Fat cannot turn into muscle and muscle cannot turn into fat. Muscle and fat are two separate and distinct tissues.
The best example I can give is when someone has to wear a cast on a broken bone. When the cast is removed, the muscles in the unused arm or leg are smaller than they were prior to the injury. If muscle could turn to fat, you should see a "fat ball" when you take the cast off. Or did you really think that casted leg or arm lost weight but your other limbs didn't?! Hmmmmmmm…. it's called muscle loss or atrophy. The muscles inside that cast never got stimulated so it simply shrunk.
In reality, muscles take a "Use it or Lose it" approach. If you don't use a muscle, it will literally waste away/shrink. This is why it is so important to strength train and use your muscles while you can since as we age we become more vulnerable to muscle loss/atrophy due to limitations of being able to move like we use to.
Muscle is your ally in the aging process.