I think u r right with this. imo age is the biggest factor.
the average age for 1st time parent is higher than ever (late 20s) but divorce rates are high too n relationships do dissolve.
From what I've seen, 1 kid is a dealbreaker or strong red flag when you're in your 20s. At the time it isn't common place. it's hard to relate when you don't have a kid. It's enough to seem like you're in a completely different stage of life n be put off the responsibility this person has. or the potential for that responsibility to fall on you. Plus, sadly its easier to take stigma seriously when u dont know members of that demographic.
By the 30s n 40s, many of those 1st marriages or srs relationships have resulted in kids n a huge number haven't worked out. More of ur friends prolly got kids. It's the norm by then. Not only r u more likely to meet prospects who r parents, but ur perception of it might have changed due to sheer exposure within ur circle. You're more likely to accept what you know.
So yes its slightly more unreasonable for a person to view someone being a parent as a deal breaker when they're 55, although I'm sure they themselves recognize by then tht they're scaling down their pool n dont need anyone to tell them that.