In reply to Monk:
> Lifestyle constraints undoubtedly have an influence, but there are also hormonal reasons. Testosterone levels drop consistently from about the age of 25, and lower testosterone makes it harder to gain and retain muscle mass and increases the likelihood of increased fast deposition.
I've had a quick google on this and there might be a mixing of cause and effect
http://www.nebido.com/obesity-is-strongly-linked-to-low-testosterone-levels... suggests
"It is now well known that there is a relationship between low testosterone levels (hypogonadism) and obesity, and the studies presented here confirm that link and show that the relationship is also true in young men who are obese.
Previously it was assumed that the decrease in testosterone seen in older men was due to age, but the age-related study showed that the drop in testosterone is actually due to weight gain, and that testosterone levels can recover through weight loss."