If you are looking to pack on some muscle size, you may want to consider adding an essential amino acid drink enriched with leucine after exercise.  Researchers examined the anabolic effects of a leucine enriched essential amino acid drink in young and old men after a bout a resistance exercise.  Researchers took muscle biopsies to examine the expression anabolic and catabolic proteins.  They found that even though the older and younger men performed the same workout (70% of a 1-RM, 8 sets of 10 repetitions), young and older men had different genes being turned on after exercise.  They found:

1. A protein called REDD2 (catabolic) which is a negative regulator of mTOR (a key anabolic pathway) was reduced in both younger and older men when they drank the leucine enriched essential acid drink.  This means leucine turns off catabolic genes.  Good news for everyone!
2. A protein called Rheb (anabolic) which a positive regulator of mTOR was only increased in the young men who consumed the leucine enriched essential acid drink.  This means younger men have greater anabolic genes being turned on after exercise compared to older men.
3. They also found increased in muscle IGF-1 levels increased only in the young, while the older men had no changes and also blunted satellite cell activity.  Older men have a blunted muscle hypertrophy response to exercise; a decrease in IGF-1 seems to be a key cause.

The results of the study suggest that an essential amino acid drink enriched with leucine which was designed to maximally stimulate protein synthesis can benefit both older and younger men after resistance exercise, but older men lose key anabolic signaling pathways compared to younger men. 

Drummond MJ, Miyazaki M, Dreyer HC, Pennings B, Dhanani S, Volpi E, Esser KA, Rasmussen BB. Expression of growth-related genes in young and  old human skeletal muscle following an acute stimulation of protein synthesis. J Appl Physiol. 2008.