"what age is her grandson" also should be accepted.
Yes. That's what I put also.
How old is the grandson of her?
No, we don't say "grandson of her". It's "her grandson" in English.
I not know
I wrote down: How old is her grandson has.:(
I've never heard anyone talk about age that way in Spanish. I would say "Cuántos años tiene su nieto". But I'm not native to Spanish. Can anyone else say with more certainty?