You're saying that if we randomly picked 22-31 year-olds, a disproportionate member would be eldest children? For that to work, there'd have to be more eldest children in that age-range than youngest. Given the increase in population, that is certainly plausible. You would expect more younger families than older families, which means that within an age range there would be a disproportionate number of older siblings (unless it's so young that not all of the younger siblings have been born yet) but it doesn't seem like it would be nearly that significant.

Many respondents' younger siblings are too young to be contributing to this site, while no one's older siblings are too old

The fact that most of the respondents are eldest children is a confounder for this.

(unless they're dead, but since ~98% of the community is under age 60 that's not a significant concern).

In that case, wouldn't people over 60 also be too old?