[cmiller1]

According to the paper this is "Fat-free mass-adjusted expenditure", and testosterone DOES start to slowly decline after around age 25. The trouble people have with starting to gain weight easier as they enter their 30s and 40s is caused by this since they'll lose muscle mass as such hormones decline, which causes fat gain assuming similar food intake and activity levels, this effect is not included by adjusting for fat-free mass.

[SamoyedFurFluff]

I also think 30-40 is more likely caused by life just being more complicated! Juggling kids is basically a full time job, and many families are two income families. Hard to hit the gym when your child, who cannot be left alone, increases your effective work hours to 80-90/week minimum no vacation no breaks.