I'm 22.
Ya know, I think it's real interesting the sort of things older people assume when talking to younger people. I must say I don't notice any correlation with age at all when it comes to wisdom, in the sense of unwise people getting wiser. Mostly what I notice is motivated and capable people simply polishing up, and unmotivated and incapable people simply getting older.
I know some middle-aged big babies, and some very young critical thinkers. Of course, I see the reverse plenty too. I consider myself a bit hard to classify, since I am way stronger in some areas than others (a side effect of being lucky enough to know what I wanted to do from early childhood). I'm not balanced enough to consider myself amongst the very best of my age group, but I certainly know people much older than me who are clearly less wise in many ways.
Basically, I think the idea that young people never know what they're talking about is bunk. And it's something said often by people who don't know what they're talking about. If they did, they wouldn't be using someone's age to try to wiggle out of the conversation.
My learnin' mostly comes from life, which then prompted personal research as to why life is this way. I'm just going to college now - I spent that 4 years traveling, by myself. And I didn't do it the easy way, either.