Hi Rivergirl
and welcome to the Forum
I havent really any advice as I had sons and they are all well grown up now with families of their own.
The one thing that occurs to me is have you met him and chatted to him. If he is a nice guy he should realise that their is a big age difference and at 21 he should know that this could worry you . Are you sure he knows her age? A friend of mine had a similar situation and the 16 year old had said she was 18 and the guy was horrified when he found she was only 16.
See if she will invite him home to a meal or a coffee so you can see for yourself what sort of young man he is ....she is very very young and personally, I would not allow her to stay over until you know more about him and his background and his life style
Good Luck
xx