For every decade we do not strength train there is approximately a seven pound loss of lean body mass for men and a five pound loss of lean body mass for women. This equates to a 2-4% drop in our metabolic rate PER decade. Even if you are aerobically active your body will still lose lean body mass if you are not strength training.
If you are reading this and realize the years have passed by without ever picking up a weight, hope is not lost. Dr. Westcott stated that there is no age limit to start strength training, just be certain you get medical clearance from your physician before starting any exercise routine.