I train people all the time of many different ages and backgrounds. I am here to tell you that you don't stop growing and producing muscle fibers after 40. Sure, recovery times tend to be longer, and lifting as heavy as you did in high school might be a challenge, but here is a list of things that you should do:

Supplements

5G Glutamine.
Protein intake 150 Grams. Mix it up between isolate whey right after lifting, and concentrate in the evening. Concentrate will give you a much fuller filling and its much slower to digest.
Get a great multi.
5G Creatine. Its a staple.
Amino Complex 2000.
Fish Oil

Stick with the this if your going all natural. Gear users its a whole different regime.