My dad has a 357 revolver that my grandad left him. That's technically a handgun/pistol right?

Since you have to be 21 to own a pistol, if someone broke into my house and threatened my life, would I be able to shoot them with the 357 (as its all we have in this house...as of now anyways...more will be coming if i have my way) even though I'm not 21 yet (erggg)?

Sorry if this sounds naive (i'm still learning)...with all the gun shots people have been reporting, I got to thinking about it. Fiend is usually here to protect me with his glock, but if I were alone, I'd like to know what I can legally do.