I don't see how age has anything to do with that. At all.You're adorable. How old are you? It's a bad idea because when there is a problem with your lover there is also a problem with your best friend. I'd bet there are stuff you can't tell him, but I'd need to know your age before I can make an argument for it.
Also, my fiance is my best friend, or else I wouldn't be marrying him. There is literally nothing I can't tell him. There are still things I haven't told my non-dating best friend.
If there's a problem with your lover/best friend, then you can either fix it or get a new one. Just because you consider your lover your *~*~best~*~* friend doesn't mean you don't have other good friends.