is there parts in the human body/brain that universally age year on year no matter how fit you are?

Our whole body is aging all the time, but our body is always repairing itself as well.  Cells are constantly dying off but we grow news ones to take their place.  Roughly, every 7 to 10 years we have replaced all the cells in our body and are technically a new person.  As we age our ability to repair ourselves diminishes so we start to lose the battle but we're all aging all the time.  Being fit, namely getting enough sleep, eating a good diet, and getting plenty of exercise, can slow down this process.

dna.           

Skin cells are the fastest aging and dying part of the human body. The brain also continues to age, hence why we died ultimately of natural causes.