While I understand that it makes sense and agree to stick with 18 for legal reasons, I just wanted to share my experience and to say that this won’t necessarily make the website lose all of the younger members.
I joined CS back in 2010, just a few weeks after my 15th Birthday, I immediately became an active member and starter traveling quite intensively. I couldn’t select an age lower then 18, so until I became actually 18, I’d just lower the age back to 18 after each birthday I had so many wonderful experiences around the world which I wouldn’t trade for anything and I could only encourage teens to start traveling young, although as long as they are mentally mature enough, can take care of themselves and their parents allow them.
Ultimately, who wants will find a way.
While that might be legally true, in practice you can travel quite young, as long as you look like you know what you are doing, I traveled alone to China and back (by land) while 17. When I was 15 in 2010 I went to around 10 countries (Mostly Central/Eastern Europe + Turkey), only the first time did I take a written and officially approved permission, but no one at the borders ever asked for it. By the time I was 18 I had visited almost 40 countries, of which at least 30 were alone.