I know this was ages ago, but it looks like an interesting thread.
Personally, I firmly believe it depends entirely on the person. I'm 16 and have been practicing Tarot for a few years, now. When I was younger, I did a reading for a boy who I thought was genuinely interested in getting a reading, only to have him mess around the entire time. It was disheartening, but I did learn to trust my instincts rather than my ego. Now, I would say, most people my age would only approach me for a reading if they were genuinely interested or had a problem they wanted help with; for those people, yes, I would read for them unless my instincts told me not to. I can understand legal issues with reading for young people- if you're charging for readings, of course, you'd have to obtain written permission from a parent or guardian before reading for a young person, I believe. However, I don't charge for readings, so I have that legal freedom to read for whoever asks me to, if I so choose.
I don't see this as an issue of "Oh, this age group is particularly impressionable," because, to an extent, yes. But every group of people is impressionable in one way or another, and I don't think it's necessary to label things exactly. I have never had a problem with people misunderstanding gruesome looking cards, or the Death card. Occasionally, I've been asked if the Death card means someone's going to die, but once I explain the card itself, there's never a problem.
Now, I hate to play a "I'm not like other people my age" game, because there's nothing wrong with other people my age. Young people take on a lot more responsibility than people tend to think, regardless of how they look or act or how many parties they go to.
Essentially, it depends on the person, not their age. Legally speaking, it's important to check out the laws about readings for under 18s, but other than that, young people as a whole are no more or less impressionable than any other age group. No one magically becomes enlightened at the age of 18- you just have to start paying taxes.