I am beginning to realize that just about everything after 50 (more specifically, after menopause) is harder to do than it was before 50! I can see the changes in my own life. But, it can still be done, with effort and dedication.
To build muscle, concentrate on lifting as heavy as you can for fewer reps. Work both your upper and lower body. As with all changes, start gradually and increase weight consistently to avoid injuries. Try the workout generator here on SparkPeople and also check out weight lifting sites, like Bodybuilding.com www.bodybuilding.com/fun/index.html
They have wonderful sections targeted to specific needs.
As Tom Venuto says, "Train hard and expect success!"