You are correct there's not that much difference between a guy in his 20's and a guy in his 30's, unless we're talking about 21 to 39 (e.g. the extremes) but your point is valid. There are, however, huge differences between people at a given age, but that's for another post.
As for this:
Where did you ascertain that this is the recommendation?The recommendation of 4 large meals and a gallon of milk a day seems less applicable to a 30 y.o. 5'9" 190 lb. advanced novice, or am I wrong?