Fine. "Young adults" it is. They can make choices and think for themselves but I would suggest that the choices they make are often poor in comparison to how they'd choose as a well-informed adult who's been around the block more than just a few times.
True. But their decision-making shouldn't be compared to children, as you did earlier. Children aren't supposed to make their own choices, so they don't. Their choices are supposed to be dependent on what adults say.
The only reason they make poor choices is because they were never taught how to make good ones. Besides, when you start talking about the poor choices that teenagers make, the least of your worries (and it's not worrisome to me at all) is whether or not they convert to or adapt a religion.
The point is, because most teenagers convert of their own free will, the idea that most religious people nowadays have been indoctrinated as children is nullified. I propose to you that indoctrination isn't what's going on with these young adults who convert to a religion: what's going on is the desire laden in most people of this age to belong to a group. (Thank the gods I didn't develop this desire until my Senior year in high school.) They're starting to develop an identity of their own, and for some of them, converting to a religion will help that. (Just like for others, leaving a childhood religion will do the opposite.)
Religion is also one of those things you have complete freedom about. If a teenager converts to a religion, and at a later date decides that it's not for him, he'll most likely leave at no cost or risk whatsoever. It may take some time and some strength to leave, but it will happen eventually. It's not like joining a gang or starting to do drugs.
Oh, and by the way, in all honesty, if you play the experience card, the only ones who can make good, informed decisions are elders who have done a little bit of everything. I work with children and young adults; I can tell you that some of them are more experienced than I am.