As you age, your body becomes less responsive to protein. The technical term for this is "anabolic resistance." It means that a person in his or her mid-70s does not respond to 20 grams of protein the same way as someone in their mid-20s. Research by Yang et al. (2012) shows that older adults (average age, 71) had a lower protein synthesis response to whey protein around their training time due to anabolic resistance. This matches other data demonstrating muscle loss as subjects age.
Before you get all bummed out, there is a simple solution. The good news is that older subjects were able to get the same response as the young bucks in the study by taking in 40 grams of protein instead of 20. In short, they needed twice the amount of protein to get the same response.
If you are older but want to maintain or build muscle mass, increase the amount of protein around your training time from 20 grams (about 1 scoop) to 40 grams (about 2 scoops).
Even if you are not over 70, there is very little downside to consuming more protein around your training time. Test it out and see how it works for you.
Reference
Yang Y, Breen L, Burd NA, Hector AJ, Churchward-Venne TA, Josse AR, Tarnopolsky MA, Phillips SM, "Resistance exercise enhances myofibrillar protein synthesis with graded intakes of whey protein in older men." Br J Nutr. 2012 Feb 7:1-9. [Epub ahead of print]