He is 30 years old, but his power-ups increase his strength greatly.
Corno was created when Bowser tried to clone Mario so that he could have a powerful force to fight for him. The experiment backfired, and Corno was created. Corno decided to side with Mario, because he didn't like Bowser's evil ways. One day, Bowser invaded the Mushroom Kingdom. Corno wanted to fight Bowser, and off he went.