It does seem awfully young but they might make a good job of it despite that, and they'll have more money to help than than many 16 year olds. My mother was 17 when she had my oldest brother and it's never hurt him or her. Good luck to them, what else can you hope?
I just hope they have the sense to go about everything quietly and sensibly, and not get caught up in some media circus which tbh will end up with people being hurtful and nasty. People forget that people in the public eye are real people with feelings, and things that are said can't be unsaid, so I hope folks are kind to them and their baby. x