Is there a mod for this?

I didn't see anything at MTS or simsasylum. You might be able to do what you want with NRaas Retuner. I've never changed these settings so consider this experimental:

City hall or any computer>NRaas>Retuner>Settings>General>By Tunable XML>Sims3.Gameplay.Objects.Environment>MagicGnome

Under this option look for these two settings:

kChanceToSpawnToddler is the chance the game will spawn a toddler gnome IF there are 2 or more gnomes on the lot. Default is 0.1 (or 10%). Try setting this value to 0 to disable the game from spawning toddler gnomes.

kDaysToAgeUpToElder is the length of the gnome adult stage (there is no YA gnome stage) in days. I don't see any way to simply stop gnomes from aging. As a workaround you could try setting this value to a large number (default is 18). Unfortunately, I don't know the upper limit for this number. Perhaps try 1000? That would be some 142 sim-weeks, probably longer then you are realistically going to play a game.

Again, I've never changed these settings so don't know if there might be any unforeseen consequences. You'd have to test it out. And sometimes the game doesn't honor xml tuning changes even if you implemented these changes in a tuning mod instead of using Retuner.

@TreyNutz Thank you! I will try that. :) I wish they had made only the plain magic gnome age and have babies. The Voltron took too long to put together just to have it die in 18 days. :|

I don't like gnomes dying when I'm trying to collect all of them either, so I just cheat a little. When I find a gnome headstone, I find out what kind of gnome died, then use the buydebug menu to replace the headstone with a live gnome of the same kind.