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.
Story Behind the Song Nelly (Feat. Tim McGraw), 'Over and Over'
I'd call all three of those slang, only to be used very informally. I've never seen yus or yos, i've only seen yas. However, it is used most common online.
