A sign that you are overloading your muscles is the lactic acid burn that you feel in the last few difficult repetitions during resistance training. The lactic acid is what signals your body to stop and rest. It is the other forms of pain that you need to watch out for, as they could mean injury has occurred. Efforts should be made now, whatever your age may be, to preserve and optimize every bit of muscle tissue that you have. As we age we naturally lose lean muscle mass, which explains why people in their 80's often walk hunched over and shuffle their feet.
Resistance training helps to combat the loss of muscle.
It could mean the difference of you getting yourself off the pot at age 80, or someone helping you get off the pot. Everyone should do some sort of resistance training. Weight rooms are no longer for bodybuilders and powerlifters. At my gym, women dominate the free weight room from 9-11am, and I love every minute of it. It is not uncommon to see teenagers lifting weights.