Hmm, well logically, if I was a man aged 35-45, and I wanted to meet a partner to have children with, I'd probably be looking for someone sub 35 to increase my chances of meeting someone, having time to get to know each other well and still be comfortably in the window where pregnancy was statistically likely to happen. So I'd probably rule out someone late 30s because by the time I'd spent a couple of years getting to know them, moving in together, getting to a point we were ready to start a family, someone I'd met when they were 37/38 would probably statistically much more likely to have fertility issues. On the other hand, if I was a man aged 35-45 and I knew that I didn't want a(nother) baby, logically I'd probably look for someone either 40+ or for someone with a completed family, as those people would be more likely to be on the same page about not wanting children.
Of course that's a logical, calculated view, but when online dating you have to have something to filter down by.
In real life, I think that the spark, the attraction, the gut feeling, the fancying would be far more important, and its more likely that you'd meet someone compatible, because you wouldn't get filtered out.