What's normal to one cat isn't normal to another.
I have a 16 year old Flame Point Persian and he must sleep 18-20 hrs a day now. Even when he was younger I noticed that he slept alot - not as much as now - but he certainly slept more than my RB tabby Speckles. I would say as long as his input and output are good then there is not too much cause for concern. If he starts to hide or becomes lethargic, that's the time for a white coat visit.

Would love to see some pictures.