There is already a way to freeze the growth of your pet's image, but what about its numerical age? 
That's what the Timestopper will do. If you want your pet to stay eighteen forever (for example, if I owned the pet BellaSwan from Twilight. This is a horrid example, but she is a vampire and she is eighteen, so I want her to stay eighteen forever) you simply use the Timestopper. It halts your pet's age.
If you would like your pet to continue ageing, simply use a Timestarter. 
These could perhaps make an interesting new pair of CS items.

@Blue cheers!

Yessss 

@jayni