If you are over 50 your ability to build muscle is severely hampered by the normal aging process.  The good news is that yes you can build and maintain muscle, but sadly you will find it takes more to achieve...less.
I weigh in at137 pounds. At my heaviest I was 155 -158. I was never a big guy, but I did have some obvious muscle and looked strong. Now I look slim and generally toned, guess I should be happy with that.

At my current body weight I can do a set of 6 or 7 standing alternate dumbbell curls with 40lbs in each hand. This is a fair amount of weight to handle for a smaller guy. Yet this does not build my arm size much, if at all. I do a great arm routine about once or twice a week, one that would grow the biceps size on any younger guy. For me it maintains what I have and keeps my arms toned and strong...but size is not happening.
Part of this is due to my smaller frame, but a lot is to do with my age.

Important for any aging strength training...remember to set REALISTIC goals.
Use creatine and protein to spike up your muscle gaining ability, taking some before and after a workout as directed. But don't hope for any incredible packed solid muscle to cover you like it might have if you were 30 or 40. You can look and feel great but be realistic.

The important thing is to maintain your strength, bone density and energy level. Hobbling around with bent legs and back due to a  lack of maintenance on your only body.... is no way to live.