You can build muscle size and strength at any age. I have done it and am now doing it at the age of 72. There are thousands of people worldwide who are maintaining and even improving on an excellent health and fitness level. Many are still increasing their muscle size and strength and endurance level. However, the training methods used have to be constantly modified as you grow older.
Organic health is a direct byproduct of heavy exercise.
The function of every bodily process is improved as the external muscles are toned enlarged and strengthened by gradually increasing the exercise workload. The combination of heavier weights, sets and reps works its magic on your internal health as it does on your external appearance and functional capabilities. If you have already been doing this for many years, don't stop after fifty. The years your body needs heavy exercise the most are after fifty. There are two groups out there, reading this.