At Character creation you can give your character an age - but this doesn't seem to show up later on the character sheet so I'm wondering if it has any effect whatsoever in game?

Is it a variable that can be referred to in conversation like <charname> or something?
e.g:
-"How old are you?"
-"I'm <age> years old"

Has anyone encountered a reference to character's age in the official campaigns? (or if not, even in a fan-made module?)

Manual doesn't seem to say and couldn't find it in a search. Just curious.
(mayhaps I'll make a 99-year old Human-Barbarian sometime)

Probably that's just because the game wanted to be the closest possible to the pen-and-paper D&D experience and age is something so simple to implement in character creation. :)

fair enough (but then you'd think it would also be as easy to show it on the character sheet after creation - I'm just wondering if it ever actually shows up anywhere in game or whether it's one of those things that they put in and never implemented beyond that creation screen).
I managed to find a webpage about scripting for NWN and there's a 'GetAge' command but I'm not sure how that would translate into 'Show Age' in a conversation and I couldn't find the relevant variable to use it.

*shrug* not that important (though I had a subsequent idea of making a 9-year old, club-wielding Barbarian named 'Bobby' (D&D cartoon reference for those who never saw it) but then the 3D-model would still look like an adult unless I made him a halfling)

Actually if your DM has a sense of humor he can change your appearance in-game using a wand or specially scripted placable item.

Ok, thanks all for the info, curiosity satisfied :)