How old is Splinter?

[Sage Ninja]

I was wondering just how old (people years) Splinter is. I mean right now he looks pretty old and decript but he seems to be very viril and just as active as his sons. At first I thought he was really old because his fur is grey/silver but in the flash back episodes when the turtles where little kids and he was considerably younger his fur was also grey/silver and he looks exactly the same as he currently does, so maybe the grey fur isn't a sign of ageing but its natural coloring, which could indicate that he is still reasonably young. My guess is that Spinter is about in his 50's in human years which isn't that old.

Although when Slinter was just a rat before he was mutated his fur was more of a tawny color other than grey , so maybe being mutated had an effect on his fur coloring.