In August, UK television personality Kirstie Allsopp found herself in an unexpected media storm after a series of her social media posts describing her 15-year-old son’s trip through Europe went viral.

“My little boy has returned from 3 weeks inter-railing,” Allsopp wrote on X. She explained that her son, Oscar, and his 16-year-old friend used the European train pass Interrail to travel to cities including Paris, Brussels, Amsterdam, Berlin, Barcelona and Madrid.

Allsopp closed the thread by stating “if we’re afraid, our children will also be afraid, if we let go, they will fly,” and tagging @freerangekids, the handle of Lenore Skenazy, a US-based writer and proponent of free-range parenting.

The original post on X now has 2 million views, and while many commenters staunchly defended Allsopp — co-host of the real-estate reality show “Location, Location, Location” — others expressed disapproval about her decision. The controversy made headlines in the British press and, according to Allsopp, even caught the attention of UK social services.

The hoopla has mostly since quieted down, but the incident has also sparked ongoing conversations about the main underlying question: What age is too young to travel without adult supervision?

According to travel insiders and safety experts, an exact number is impossible to pin down, since it depends on a wide range of factors, from the maturity and experience of the traveler to laws about minimum ages to the safety of the destination.

“I know 50-year-olds who are feckless and couldn’t be trusted to go to another country, so I don’t think it’s an age [issue],” Carolyn Pearson, CEO of Maiden Voyage, a firm that specializes in helping business travelers stay safe while traveling, told CNN Travel. “I think it’s a maturity and an outlook which determines whether or not somebody is fit to travel. It’s down to the individual and the caregivers that a mature decision is made on the suitability of the person and the suitability of the place that they’re traveling to.”

Pearson said the controversy surrounding Allsopp was the subject of “quite animated” household debate with her partner (until, she said, he learned that Oscar’s trip wasn’t illegal, leading him to agree with Pearson that it was “a non-story”). But as the founder of a company that teaches people how to travel safely, Pearson also understands that safety is a main concern for parents considering whether to let their children travel without them.

“I’m all about empowering travelers,” she said, “and I think also the life skills that someone’s going to learn in three weeks traveling by themselves is probably going to be more than what they’re going to learn for three years in school.”

From a legal perspective, Oscar’s European adventure did not appear to violate any laws. He and his 16-year-old friend traveled to several European Union countries with a type of train pass that enables passengers to take advantage of the bloc’s open borders. (The Eurail pass provides similar accessfor travelers residing outside the EU.)

Interrailing and Eurailing, as this mode of train transit is often known, are popular among young people aiming to visit many European destinations before starting college.

In an email to CNN, an Interrail spokesperson confirmed that minors can purchase Interrail and Eurail passes. They also emphasized that travelers should always check the Conditions of Use beforehand regarding specific requirements “of the exit and destination country,” noting that it is “the sole responsibility of the traveller or their legal representative to verify which rules and restrictions apply and ensure they are followed.”

Mark Smith, an expert on train travel who runs the site The Man in Seat 61, also points out that “in many cases there aren’t any formal rules” around age limits for train passengers, though the sector has become more controlled than in decades past, especially as some airlines have paired with train operators for ticketing options.

An official European Union website for travel regulations is notably ambiguous, stating that some countries may require a consent letter for minors but also that “there are no EU rules on this matter.”

Channel Tunnel train operator Eurostar states on its website that children aged 12-15 years are allowed to travel to certain Eurostar destinations, but they must have a completed consent form and only travel on trains departing between 6 a.m. and 5 p.m. There are additional requirements for French residents, and passengers under 16 are not allowed to travel unaccompanied by an adult on direct trains to or from the Netherlands.

Meanwhile, US train operator Amtrak specifies on its website that children aged 13, 14, and 15 are allowed to travel as unaccompanied minors. However, an adult must be present for departure an hour beforehand and sign a consent form. Station personnel also interview the minor in question “to determine if the child is capable of traveling alone.”

Most hostels – the accommodation of choice for many cash-strapped younger travelers looking to stretch their budgets as much as possible – have some guardrails in place around unaccompanied minors.

An employee from Hostelling International, the world’s largest youth hostel network, confirmed to CNN Travel via email that minors under 18 years of age need a completed consent form from a legal guardian to stay at properties within its 60 member associations. That said, there’s no network minimum age for guests, though  member hostels may have their own rules about age requirements.

On the hotel front, many major chains have a minimum age of 18 or 21 years for guests, which is often listed in a brand’s FAQ page, along with other requirements.

But there are gray areas, too: Hyatt, for example, states that “generally, the minimum age to reserve a guestroom at Hyatt is 21 years old. However, this age may vary by hotel,” and encourages guests to check ahead of time regarding policies. A person who meets the minimum age requirement also must be present at check-in and is required to be a registered guest.

Hilton, meanwhile, has no standard policy but recommends on its help page that guests check with individual properties.

Proponents of allowing eager young travelers to explore the world at ages that might raise a few eyebrows say the benefits can outweigh the risks.

Cole Robinson, a documentary filmmaker based in Manchester, describes his first solo trip – to Barcelona at age 16 – as an “immensely pivotal decision in my life,” which inspired him to pursue a career that would allow him to travel the world.

After convincing his parents he was responsible and ready, Robinson was off. As soon as he settled into his hostel in the city’s Eixample district and began getting to know his fellow travelers, Robinson’s passion for travel was ignited.

“Just sitting in that communal area in the hostel and chatting to people from all over and having conversations that I found so interesting, hearing other people’s perspectives and about the way they live their lives, I think it’s still with me,” Robinson says.

Robinson continued traveling, first with friends to more European countries and then trekking around South America solo. “That was my goal from ages 17 to 19 – go to as many places as possible,” he said. Robinson began making videos from his travels, which eventually led to him founding a video production company, Filmit UK.

“Travel, it throws up so many difficult situations, and there’s so much unpredictability,” Robinson said. “I think especially for a young person to navigate those challenges and do so well, it’s extremely beneficial to their development.”

On the flip side, some experts contend that just because a teenager is legally allowed to travel without adult supervision doesn’t mean they should.

“I think age 18 is a good guideline for a long trip without adult supervision,” said Jenny Rankin, a Southern California-based educational researcher and author, via email. “I can imagine dropping this to age 17 for a responsible youth like [Allsopp’s son] Oscar, but I feel 15 is too young for an extended teen-only trip.”

According to Rankin, a big part of the challenge boils down to still-developing brain function in teenagers. A region called the prefrontal cortex, which affects impulse control, emotional regulation, and decision-making, “lags behind other brain functions during teen years and is still trying to catch up,” Rankin explained – which can be especially problematic in unfamiliar environments and situations.

“Even a 15- or 16-year-old boy who makes great decisions in non-emotional areas, like solving algebraic problems, is prone to sudden, unexpectedly poor judgments when influenced by emotion or impulse, like whether to follow an attractive teen down a dark alley or run across train tracks when trying to catch a ride,” Rankin said.

Cultural differences, too, play a major role. “In the US it is commonly accepted that European teens are more independent and embraced into adult culture earlier, like French teens commonly drinking wine at family dinners,” Rankin said. Beyond a younger legal drinking age – 18 years for most European countries, as opposed to 21 in the US – mass transit in Europe and many Asian countries is far more robust, which makes transportation much more convenient and safer.

For Jonathan Alder, founder of award-winning agency Jonathan’s Travels, a memorable moment highlighting those cultural differences happened during a trip to Japan with clients in September 2023.