Well, this is third-hand information at best (that is, someone said that one of the game developers said it. I don't remember where I read it, sorry), but the official line seems to be that the main character, Imoen and Jaheira are all 20 at the beginning of BG 2. My personal opinion is that this is a bit old for Imoen, a lot too young for Jahiera and complete rubbish for any main character who isn't human*, but that's what I've heard. This would mean either that Gorion was in the habit of sneaking out of Candlekeep to adventure with the Harpers, or that Jaheira started her career at a very early age.
I also seem to remember that David Gaider has said that Viconia is "a lot older than Drizzt"**, which is probably a bot more useful if you know how old he is...
No Idea about Aerie, but she's probably older than 20.
This means that there is an age difference in at least two cases, but I don't really see that as a problem. It's not like the main character is a child or anything, it's just that the age of potential love interests registers on a different scale.
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That's right, this post has footnotes. Just because I can.
*These days, I solve that problem by only playing human and half-elven characters just to avoid the glaring inconsistency, just like I avoid playing monks or barbarians or any other class not likely to grow up in Candlekeep
**Try lookig for that in the "Official information" thread on the Unfinished Business forum. I think that's where I read it.