I run a 3.5 dnd game. Two of my players have animal companions they fight alongside. These pets will soon be too underpowered to bring into battle with them. My players have spent years, in game time, training their battle buddies. Anyone have house rules on leveling up pets. I can't find any official rules on it.
Are these classes that have animal companions with them, or is it a fighter that has a trained a wolf to fight by his side and help trip opponents?
This is an important question from a game mechanics viewpoint. Some classes have built in rules for animal companions and leveling them up with the character (in 3.5 I believe this is a Druid thing, but I could be remembering wrong). Having this built into the class allows for built in game balance. That is, the class may get fewer other benefits as they level up, because they have a more powerful companion to help them in battle.
I'd say if these are just animals the players have (such as a horse mount or a trained dog/bird), keep their base stats. Just because the players have entered a level range where they are gaining god-like powers and facing really powerful enemies, doesn't mean the horses they ride are suddenly super-horses.
It's both, but it's mostly two trained animals they fight with. For example my halfling wizard has an owl familiar but has had a mastiff watching his back for a long time. I see your point about your regular trained animals not becoming super animals just because you're becoming super powerful, but these animals have been fighting along side these two guys for a while that's gotta count for something.
As friartook mentioned the classes that have it built in are using that to gauge how much more the class is given at each level. The way to "help" balance things is to make sure the players that want to upscale their normal animal are paying for it through gold or work/quests. This could be paying to have a wizard or druid leveling up the animal to it's advanced form, or buying armor, or protective gear for the animal. I'm sure that there is a book or twelve out there for gear specific to animals given that you are playing 3.5. I know the feeling, but the other thing is running the risk of the animals dying and getting to deal with that as a DM, dun dun duunn.
That is a good idea, some armor for the different animals would go a long way to stretch the lifespan of their pets. Plus it would be kind of cool picturing my ranger going into battle with a tiger dressed like battlecat from He-Man. I sold my paladin some leather barding for his horse. ( I made him pay 2000 gp for it. There is a funny story behind that.) But It never crossed my mind to armor up a dog.