Ageing is not a matter of counting birthdays as of changes in fitness; in the way your body works and reacts. If your body changes enough that you look, feel, and function differently than when you were younger, age may be overtaking you.
Chronological age and biological age is not the same. Ageing is a physiological process that at times is only remotely connected to how old you are. How you look is sometimes an indicator of you biological age, but appearances often can be deceptive.