Since you're a newish player, I don't know if you know about this, but there's something called a Regression Ray (rray). This allows you to zap a pet to any stage you want (baby, teen, or adult). They are retired, but many users have one and you can either borrow it or trade your pet to them to have them zap it and return it. This is a common practice among users. Many will do it for free while others will charge you for it. However, the charge is never a lot, I've never seen it over 25k. Usually it's more like 10-15k.
Having an option like this would devalue the rray, so when it was suggested before it was turned down.
This is only applicable to pets you want to keep on your profile anyway. Pets in your showroom don't age, so if you want to have a pet stay the same age and it doesn't have to be on your profile, just shove it in your showroom. Pets in rancher shops don't age either but unless you put it at a ridiculously high price people either won't be able to see it or might buy it, so a showroom is better for storage.