Hrm. Make sure you have updated all your hacks or have Pets compatible versions of your hacks. If you have any woohoo related hacks, particularly my risky woohoo, I updated that for Pets recently. Go get the latest version.
As for aging... do you have the Age Duration Hack? If so, make sure you have the most recent version. twojeffs has also updated that.
Otherwise, I don't know of any hacks, off the top of my head, that might cause breeding or aging problems with pets. If you are using Squinge type stuff, I would go to the InSim site and make sure all the hacks that you have of his are compatible or have updated versions.
Ste
The only woohoo hack I have is Casual Romance the autonomous version, I download the latest ones as soon as they come out before playing my game so it can't be that. It was working before with that one in there anyway.
I also have the latest age duration which was updated as well. I'll try getting these hacks again incase something happened and the latest one isn't in the game. I'll also go through the hacks I have by squinge and see if any of them in my game aren't compatable. None of them seemed like a possible conflict but I'll check them anyway and see what happens.