I think that teens can easily get away with it, considering that that's when most people do stuff like this. I mean, I dyed my hair bright red when I was 16/17. So I think it's fine, as long as you take care of your hair well and make sure you put effort into maintaining the color (nobody thinks that faded dyed hair + crazy long roots + massive split ends look good). If you feel like you'd be too lazy to maintain the color by dying it at least once a month, then a bright color is probably not for you. Bright colors look the best when they're bright and freshly dyed. Not when they have faded and your roots are showing. So, it's quite the task to maintain it. I guess that's why dip dyes are a bit more popular, since you don't have to commit as much. Anyway, yeah, as long as you put effort into it, I think brightly dyed hair can look really cool. If you want to do it, then go for it.
I love it! But I never keep it for long because of the attention it attracts. ( I don't do it for attention, I do it because I think it's sexy) but once or twice a year I do red, then go back to my natural black after a month or so.
It depends on if the person can pull it of or not. I've seen some horrible dye jobs that make me think, 'what the hell were you thinking', and some others that I couldn't imagine that person any other way.