She's like 22, and objectively cute and goodlooking.
I don't see what the problem is given how Mike is 28, but I must admit, she does seem really immature, and I wouldn't be surprised if she is playing him. But hey, that's his problem and not ours.
Wish them all the best, and hope it's not another toxic relationship. Mike seems to wear his heart on his sleeve, and this usually gets abused.