As the title says.

I believe it's only resting and certain effects that specifically alter aging.

In Wizardry 1-5, it works like this:
Resting at the Inn will age just that character. (In the Apple 2 versions of 1-3, this source of aging wasn't implemented at all. Note that you can recover your spells for free with only a 1 in 7 chance of aging a week, and healing spells exist so you don't need to rest to heal up.)
Changing class ages your character significantly (something like 5-7 years). (Note that this doesn't happen in 6 and 7, which handle things very differently.)
Being revived from the dead may age the character a year, depending on the method used.
In Wizardry Gaiden 4 and Wizardry DIMGUIL, when you create a character who has less than a certain number of stat bonus points, you are given the opportunity to have the character train 3 years. Doing so ages the character 3 years, but in return the character gets to level up to level 4 right away.
Invoking an item can sometimes change the character's age, but will not lower it below 19.

this is something that i always wondered on Wizardry 7 (the game i started to play Wizardry)... i had to rest ALOT constantly (i think mainly due to inefficient play on my side).
And i never knew if my chars actually would age as much as they rested and eventually die a natural death due to their age.
Needless to say that i never got that far to actually find out..however i assume some of the buffs can actually clear this up for me.

#my RL character aged about 23 years since I initially wondered about that :D

I was wondering about this a while back for Wizardry 7, so I cracked open one of my ancient saves in the Cosmic Forge savefile viewer, which shows a lot of information that even the savefile editors don't show. This particular savefile was my very first completion of the game, back in the 90s when I didn't actually have any clue what I was doing with the game but I loved it anyway.

The ingame date was 674 days. So that's less than two years spent, even with loads of resting and loads of backtracking. Since all of the vitality-boosting items reduce your age, and no character starts at an age higher than 20, and such relatively little time passed even for an extra-long playthrough, I'm not surprised that aging isn't really a thing. It would need some sort of game mechanic to age you faster in order to be relevant, and I don't think one exists in Wiz 7.

Honestly I'd be surprised if aging even has any code to go with it in 7, considering how difficult it is to actually increase your age.

Well, you could use an editor to increase a character's age.

Try setting a character's age to 130 and then letting the character level up. Do the stats still increase, or do they decrease?

It's interesting that Wizardry 5 has a spell (IHALON) that can decrease a character's age significantly when cast. I suspect that the developers wanted to move away from the Age mechanic, but didn't feel like they could simply remove it, for whatever reason.

Thanks for shedding some light on that bevinator and dtgreene.
Given how difficult Wizardry can be at times i assumed they might have even included dying due to being deadly old even ;D
As there is no indicator on how long you rest , apart from the day night cycle (whatever that may mean on the planet of guardia) i never figured what consequences of my prolonged resting might turn out like.