He's not a real grandpa, but he's old enough to be one.A little over 50 according to Avatar Wiki. Not a grandpa by any means, I mean he has little children.
Most common age to have kids is mid 20s so by the time you're 50 you could have kids who are 25 themselves and already having their own kids.
By the time you're in your 80s your grandkids will be fully grown and ready to start having kids of their own.
It looks like Tenzin didn't have kids until he was already like 45, which is pretty late as things go.
Regardless, the point is that he's a old man.