Well, you must be
Iron!
I'm 26
The element Iron has an atomic number of 26.
I'm usually cool
Iron is a metal, typically cold to the touch.
Sometimes I am hot, steaming hot
I expect this is referring to an iron that you would use to iron clothes, which are incredibly hot.
The Male me have a shining heart and can fly
This is clearly Iron Man, who has his suit featuring the glowing core and the ability to fly with thrusters!
The Young female me is, however, horrible and brings a lot of pain
Well I'm guessing this is referring to Iron Maiden, but I don't know why they're so horrible - they do sing about painful stuff, though.