They are, what, 22 or 23? And they look fine. But there's nothing wrong with Barbara the elder. She has, by all accounts, been a fine wife and mother who has aged gracefully. Her snow white hair is just like my own mom's (she is 78, it was a sort of dark auburn when she was 30, started greying when she was about 35), and I, for one, like it.
I have almost no grey at 46, so for that I guess I am lucky (but then again I also have almost no hair).
Mrs. Bush went grey almost overnight when our president's little sister died of leukemia. She said that our president would tell his friends he had to stay in and take care of his mother (after Robin's death).