Well, in White Knight, Luccio says:
Maybe when you're a wizard, your body sorta works like everyone else's until you become a "senior citizen" and then you just continue to live. So, your fertile period is the more or less the same as any other normal human?
I don't remember how old Luccio is supposed to be. Also, I think Harry (double) implied that she was a very experienced person in the bedroom, so, sounds like she used to have a very healthy sexual life.
But then, Margaret was over a hundred when she had Harry, right?
It is a bid weird, the lack of children from McCoy. Maybe this family is just repressed in general, LOL.
Honestly I think this was a flaw Jim hadn't considered when younger body swapping Luccio in Dead Beat which we know had some divergence from the original plan of the series.
I don't think Jim had considered that Margaret le Fey was already over a century old when she had Harry and Thomas. One of those things that could have slipped his mind when writing for the Hardback Dead Beat.
