I have a little 6 year old Shih-Tzu, well 6 on Oct 3rd. He loves a race around, but not for too long, will happily walk several miles and has no problems up hills or steps. He does sleep a lot, but also plays a lot too and is a great little watch dog. He's a great little fellow. He is my first 'little' dog as I previously had Irish Wolfhounds, an Irish Setter and a St. Bernard (not all at once !... :o). So, I am not an expert on Shih-Tzus, but I would say that you should talk to your vet about this. It would seem to me that they are rather young to be like this. Perhaps they need a bit more stimulation. I would go to the vet, rule out any medical problems and then work with them to get them motivated again. Perhaps some clicker training exercises, which most dogs and owners really enjoy. Like humans - doggies can get just plain lazy. But if you want them to stay healthy they will need exercise and stimulation. The younger one might just be adopting the bad habits of the older dog. Perhaps a change in food or toys might help too. Good Luck!