I didn't find pregnancy at 38 that much more draining than at 28, but the one at 42 was noticably harder on me than the previous two. So yes, I do buy that he's noticing how much harder it is to deal with parenting a small child than it would've been ten years earlier.

And as a GenX married to a Boomer whose kids' classmates' parents are mostly Millenials? Yeah, hanging around with the other parents can make you feel old. It's surmountable, if LW decides he wants to get over it and learn to cope.