You put on said item, and assume whatever new age. Remove it, and go back to being whatever age you were prior.

For simplicity we'll pretend you've got an item that adds 10 years, and you're a human that is currently 16. You put on that item, and you become 26. So, you go from Young to Mature. That part is permanent. If you take off that item, you're now a Mature 16 year old.

Basically its only permanent if you go up an age tier, which is why it becomes really hazardous to wear +age items when your character is already pretty old.