This article from Harvard Medical School (
Harvard Men's Health Watch : Preserve your muscle mass) recommends:
1 to 1.3 grams (g) of protein per kilogram of body weight for older adults who do resistance training.
This article from Cycling Performance Tips (Building Blocks of All Foods : Protein) has lots of information about protein requirements, including:
Taking both aging and activity level into account, an international panel of experts recommended protein intakes of 1.0 - 1.2 g/kg/day for all adults 65 years or older, with even higher intakes for those who are more physically active.
I'm going on 61, and I ride about 15 hours per week. I get my protein from egg whites, soy milk, whole eggs, and yogurt. But based on these recommendations, I'm still not getting enough. That might explain these tired legs.