Of course there's a risk; there's always an element of risk in travelling. There's also an element of risk in just going outside your front door. Or never going outside your front door.
You can take a certain amount of action to mitigate the risks - e.g. take all your travel jabs and anti-malarials, make sure you've got a basic knowledge of first aid and so on, as well as basic stuff like not walking round with a bulging wallet overflowing with notes in different currencies half out of your pocket or something really daft like that. Travelling with a group will generally be safer (and easier), and there are plenty of registered charities and other groups.
I couldn't afford to go travelling till I was in my late 20s, but I'm not sure there was that much less risk in the travelling I did do before then, which was mostly cycling round bits of Britain, quite often on my own, and I was doing that from 16 (to be fair, my mother later said she can't imagine how she allowed it at that age, but I always had permission at the time, and they knew my plans.)
If young people are going in a group, then they're as likely to be as safe as is possible. If you've done a good job at bringing them up, they should do okay. You can't remove all risk, but avoiding tricky situations, or getting yourself out of those which turn out to be unavoidable is part of gaining maturity, and ideally, it's just the next step on from where you've got them to in life so far.
But there are many, many different 18 year olds...