Can a 55 year old male become as fit as a 30 year old male?

Define "fit"... BMI? Strength? Endurance? Speed?

A 55 year old can certainly lower his BMI to whatever he wants.

A 55 year old can also build muscle mass and strength if he is dedicated.

A 55 year old can improve his endurance but will probably have trouble matching the natural ability of a younger man.

A 55 year old will probably have trouble developing as much speed as a younger man.