I just remembered, when I was about 19 and catching a bus to uni, a very creepy old man sidled up to me at the bus stop and started talking to me, touching my arm, etc. I tried to ignore him and move away but then the bus came and we got on - and he sat down next to me. I very loudly said "I don't want you to sit next to me", and got out of my seat. Not one adult came to my assistance - and I am a very slight person - and in fact they looked away and avoided eye contact. I was quite distressed but nobody wanted to help.
Also, I was at work late one evening - we had an open evening. I was rostered to stand in a fairly dark part of the school, away from all the action, with three Year 12 girls to welcome people to the evening. An older man approached the girls and started chatting. Because they've been raised to be polite, they talked back to him but it was clear they were uncomfortable. When I intervened it became clear the man had no interest in the school, and he quickly disappeared. My point is, kids may be old enough and trustworthy enough, but they are still vulnerable and don't always have the experience to deal with difficult situations. Adults need to look out for them - I'd like to think people would look out for my boys.