Hello. 20 months old is not unusual for a hammy to start showing signs of aging. My Patches is 22 months old and is clearly slowing down, not grooming herself as she used to and has some hair thinning. But the only real way to be sure that a hammy is just going through the aging process, and not having a medical or parasite problem, is to have him seen by a vet. It's the only way. Hammies who are seen at an early date by a vet when a problem is noticed tend to do a whole lot better, even if the problem is serious, and the hammy has a much better time of it, in terms of actually feeling ill. Why wait. Have your hammy checked out and at least know what you are dealing with. Good luck with the little furry
P.S. I just adore the name Hamburglar.