It didn't bother me when our boy (technically my stepson) turned 21. It bothered the Boss.
But this one is different.
You have to realize that my first wife decided she didn't want children. Before we were married, we'd talked about it, and she said she wanted to wait a few years until her career got going... after the fact, her attitude was "kids? I don't want kids! When did I ever say I'd want kids!?"
So as life turned out, LMFD will be my only biological child. Not that our son means any less to me, but to some degree, she is special.
And, he feels special too. His father also has remarried, and he has a brother & sister on that side of his family, but he is the only one with both of his parents. So don't think he gets left out. (Poor guy, both of his step parents, as it turns out, are Jewish, and both of his parents are Catholic. He really does get the guilt trip from all ends!)