Mix It Up!

When you stop gaining strength and mass, dont do more of the same but instead mix things up. This idea is not new and has gone under many different names: "muscle confusion", "confusion principle", etc. The basic idea is that if you always do heavy weights or always do light weights, the muscles adapt and stop growing. Find some way to vary high weight low rep workouts with low weight high rep workouts. 

You can vary it within the workout or you can make entire workouts low rep or high rep. Personally, I favor the latter. I am an advanced bodybuilder so I workout each bodypart once a week so I have adequate recovery time. Since I am older, I workout three weeks in a row with low weight high reps and on the forth week I go heavy with low reps. The reason that I do more high rep workouts than low rep workouts is to reduce the chance of injury. The older you are, the more likely it is that you will suffer from overuse injuries or other problems. 

Here is what I would recommend as a starting point:

Age	Monthly workout schedule per bodypart
20s	3 low rep workouts, 1 high rep workout
30s	3 low rep workouts, 1 high rep workout
40s	2 low rep workouts, 2 high rep workout
50s	1 low rep workouts, 3 high rep workout
over 60	4 high rep workouts