I have had two of mine head off on gap years.
For a start at 18 they are young adults not teen. The use of teen by the sponsor of this thread spreads 'worry' in my view.
Mine researched countries and places with friends and made their own decisions. Mine are used to travelling for themselves - trains, buses etc in UK, but also across Europe on flights, trains and buses. All three have travelled independently for holidays in the UK, and for a few weeks/months in Europe for summer jobs, from age 16.
One headed off to Japan for 4 months three weeks after 18th birthday, then over to NZ and Aus for the rest of the year. He is currently working in UK and saving to do a North-South of South America, and he might do it by bike. His best experiences were the amazing variety of people he met in hostels across Japan - from Spanish to Columbian, Kiwi's to Irish, families, couples and singles. He loved that mix of people (he is very people person). With those random people he had some really amazing experiences 'off the beaten track'. In Aus he and a pal bought the most banger of cars I could imagine - and learned to mend it and keep it going for 6 weeks of travel, including a minor accident where they offered sweat labour to local garage to fix it...what an experience.
One is two years into NZ on youth working visa, about to head to Canada and likely back to the Alps. Tomorrow he heads off on 6-week bike tour of NZ, then another winter of work at the ski resort. En-route he dropped in on a cousin in Uganda and the pair of them travelled Uganda for a fortnight on buses. He is thinking about a stop at a pacific island on the way home before Canada with his new OH.
Both have WhatsApp and agreement is a weekly check in to remind me they are alive...
All have paid their own way, found jobs en-route and had to budget hard. They have had to find accommodation, problem solve issues when they arrive, met a dizzying variety of people, had opportunity to go places and do things spontaneously, change flights or travel dates, learn how to keep safe, cook for themselves and so, so much more.
On an organised year, I don't feel they would get such a rich experience. You stay in pre-defined accommodation, you meet similar (wealthy) UK folk and spend more time with them than locals or other nationalities, you are not able to be as spontaneous, you don't have to deal with issues as there is someone else doing the 'helicopter parenting', you are not budgeting and cooking so much.
I am a huge advocate of a year out - mine grew so much through it. But I do feel that companies like the sponsor of this thread do feed off parental worries, cater for a market of ill-prepared and immature 18 year olds who, with some thought and effort, could find their own cheaper, richer experience.