Can a 14 year old step grandson sit with an elderly grandparent?

Claudia, it all depends on the maturity level of that grandchild. Some at 14 are quite grown up, and others are still doing stupid things that could get themselves and others hurt.

My only concern is if a grandchild is caregiving, what are they missing out in their own lives, such as being with friends, doing kid stuff, etc. Time flies by, and ten years from now I wouldn't want to read that that grandchild is now 24 and doing full-time caregiving for his grandparent.

I don't see why not. Is there a problem?

Why does Mary need someone to sit with her? Would Jayden know what to do in an emergency? And does a 14-year-old boy even want to sit wit an old lady?

We used to stay with my grandfather all the time when we were teens! Of course as the others have said, it all depends. How long? How far away is help if it's needed? It sounds like she is pretty independent, much more so than my grandfather was, and all she needs is someone there to help out a little. What a great bonding experience for both of them, these are memories he will cherish someday!

I think it would be ok on a limited basis. Make sure he knows how to deal with basic emergencies and he has your contact info. I wish I had spent more time with my grandparents at that age, but I was very busy being a self centered teen.

They need to be a kid at that age, just because there is limited time to be a kid.
Your question seems to indicate that he would be there at bedtime and perhaps beyond. I think that's too much, he needs the socialization that he can't get by being at home with his grandmother every day. No matter what he says.

By the way, there is USUALLY a world of difference in the maturity level of a 14 year old boy and a 14 year old girl.

If Grandma is that independent, why does she need a sitter?