Yes is okay to use informally and formally. It is incredibly informal and colloquial, and can be considered a form of slang. I'm also a 36 year old dude, so maybe.
Nelly ft. Tim McGraw Over and Over YouTube
I'd call all three of those slang, only to be used very informally.
However, it is used most common online.
I've never seen yus or yos, i've only seen yas.
