To borrow terms from RFC 2119, "having a child makes people more responsible" is a SHOULD, but statistically, turns out to be a MAY. (#survivorBias: people are likely to acknowledge this, if they did turn out to actually be more responsible - "turned out GREAT for ME", emphasis added. The other case, not so much.)
However, I feel like the age of a parent is a factor, sure - but it's not an overwhelming factor...
I will say though, I think there's a chicken and egg element in this line of thinking.
A part of my thinks that being childless in twenties provides space that facilitates being a basket-case.
I think that having a child immediately makes most people at least 50% more responsible, and 85%+ within a year.
Again, there's a huge range here for people who: - Never get better - Their 85%+ still isn't really responsible enough.
Unsolicited 2 cents from a guy who had a kid in his twenties