That's not what's implied. Only thing that was said was that the average lifespan was 31. Could be any curve under that number, but it's bad however you cut it.
Isn't that average lifespan? There was a lot of death either in childbirth or during early childhood years. If you filter that data out a bit, adults lived a more comparable age. Still not as good as today, but not 31.
TBH I'm not sure the argument that only women and those under 5 were dying young in large numbers is a great rebuttal to the idea that we were better off in 1023.