Cowgirls I would get a vet to check his teeth - I had my little horse Persil for years before discovering that he was much much older than I had thought(felt real stupid about that!)
He started to really drop topline in the winter - we had blood tests done; everything you can think of all coming back normal - very frustrating. The thing was that as soon as spring came he would bounce back and look superb again. (Also he has always been very lively and active even when he looked so awful)
Then I had a vet/horse dentist come out and this time he actually knocked the Persil out in order to do a thorough job and he asked me how old I thought Persil was. " around 15" I said. The vet looked at me and ALMOST rolled his eyes - he said "this horse is on his mid twenties".
You could have knocked me down with a feather.
Anyway, this has really helped me as now I know to be proactive before winter even starts and stuff high protein feed into him (as our grass lacks it in autumn/winter).