Is it possible for an 80 year old to have a grandparent that is still alive?

Yes, it is possible. If a girl of fifteen has a baby, and that child also has a baby at fifteen. And they all live eighty more years. The grandmother would be 110, the mother would be 95, and the daughter would be eighty. I'm sure this has happened MANY times.

No

Barely, but not impossible.

Yes

No.

Yes, if their grandparent was around the age of the oldest living person, 122. and 80 year old couldn've been born to a 13 year old mother, who was born to lets say a 13 year old mother too. The 80 year old would have a 106 year old grandparent.

Yes, if his parents and grandparents had children in their late teens. .

Barely -- a friend of mine is 80, mother is 95 and grandmother is 112 and still looks like being around for at least another 5 years.

But that's an exceptional family

Yes, watch the green mile