Proteins become less stable and accurate in their roles.
Cells don't die when they're supposed to.
The body's energy production machinery malfunctions.
Metabolism can become imbalanced.
Tissues stop getting fixed and renewed.
Cells become bad at communicating with one another.
These are the resons why humans age.
Hiko is human.
As he has lived for a long time, these problems have started which is result makes him age and is now considered OLD.
He is a mutant actully.
His gameplay's are of a inhuman
unfortunately his body is of a human, that's why sometimes he is bad as his body can't keep up with is inhuman self. (@queueK this should answer your question)