The ghosts and the Voltron gnome both age, for some reason. If you want to keep them indefinitely, you have to go into testingcheats occasionally and click on them to change their age back to toddler. Mysterious Mr. is the only other one that ages, but in his case you can see the age he is. (The ghost will change color as you change the age, but it doesn't seem to be consistent in my game, so you can't say it's time to youthen it when it's white, or whatever.)
University gnome may be harder than expected. I've never done anything with the University itself, and it crashes everytime I try to go there. :(