I have an old cat. A tabby. About 13 or 14 years old. He was super active and wild as a small cat so I don't think he'll go to 20 as some Tabby's do. Hitting 14 or so means he's old. Recent the vet noticed lost weight and to check more isn't lost. He has kept his weight, but he walks fast rather than runs the last 18 months, his back legs not sprightly. Is this all old age and normal?