you can't leave your home by yourself apparently, so gorion has to take you away.
there's no restriction on >leaving< candlekeep (apart from introduction-game-device, the idea being you're getting ready to specifically leave with gorion), it has nothing to with charname's age or Realms laws...
the mage paperdoll, which appears to be quite aged.
yes, that is a crock, as are a few portraits. or you just happen to have developed white hair/fast-developing beard. both have happened to young humans, and there's no telling what the blood of bhaal'll do to ye. customization is the true answer.
as far as races of greater longevity, you have roughly three choices: say the campaign is fudged for the sake of player's free choice of race (even though they left out half-orc, the racist scum - there longevity wouldn't be an issue!)... or say that charname is in fact very young for its race (could always painstakingly do an 'elder race' rendition of all dialog and toss a few hundred 'stripling', 'kid', et al, more interestingly tweak a few encounters where they won't be taken seriously without high CHA or being led by an older party member... and/or assume the blood accelerates all Bhaalspawn's maturation (from birth) to match the human [arbitrary? bhaal was supposedly originally human...] rate for the purposes of the Big Showdown (e.g., sendai doesn't act young, and who, even among the drow, can 'naturally' learn all of that magic in less than two decades (going out on a limb and assuming she didn't start at birth!)?)