Re: Keldorn - Possibly - he does have one daughter who is still a child (the other is Imoen's age, so he implies in a ToB banter, at any rate), so he's probably not that old. But then my boss, who is 51, has a son of 7, so I'd say Keldorn is around the 48-50 mark, possibly a year or two older, but not much more than that. The 20-something marriage is unlikely - I would guess that he threw himself into his duty until an elder relative bullied him into marrying a younger noble woman in order to beget a Firecam heir while he was well into his thirties.
Remember this is the Forgotten Realms-humans are independent adults at 15, and most humans are married with kids by 20. As a nobleman in an extremely hazardous occupation, it would be even more important for Keldorn to marry and father children early-before he's killed in battle. Paladins are characterized as having extremely short life expectancies.
Also, remember that Charname is a little unusual, being single and devoid of experience in romantic relationships at 20 or so, because he/she grew up in Candlekeep, where there was only one other person his/her age. Note that in BG2, about the only characters who haven't had experience in relationships with the opposite sex are Minsc (who isn't a man, yet, in Rashemar culture-that's what his dajemma is all about), Anomen (who's thrown himself into other things), and Aerie (who's pretty self-explanatory).