An article in Nature appears to contradict McArdle's claim that "the older you are, the more heritable weight is":

These findings suggest that adult body size, shape, and composition are highly heritable in both women and men, although a decreasing tendency is seen with advancing age.

I don't think this is McArdle's fault; there happen to be contradictory findings which have probably reported excessive confidence. An increase in heritability means that the genes really expressed themselves more strongly, or variation in environment decreased (or both, obviously). Looking at the reported research myself, I can only think "it's not safe to say whether age will result in more (or less) similar weights amongst genetically similar individuals".