I'll do the same example:

You're a 16 year old human, you wear a +10 year age item. You get bumped up to mature with all which that means (+str +wis), you remove the +10age item. Bam, you're back to 16 years old. You're no longer mature either, but the effects are lasting. (No, you won't get the affects again when you become "mature" again.)

So, the +age effect is instantanious, the affect on stats are permanent. But the age or it's tier is not.

There are however items that speed up your ageing. Especially one item that will age-die a arial or felar within a few ticks.

Always shoot first and then call whatever you hit the target.