With age muscular can become weaker . this is due to

Sarcopenia is the condition in which muscles become weak because of aging. 
This is normal in all human bodies because it has the tendency of slowly deteriorating through time. 
This condition cannot be prevented. 
Muscles, joints, and bones start to weaken and lose its functions as we age.

Ithink it true i might be wrong so don't take my word that its true.

They break down fat