The second thing ("watashi wa furui desu") is wrong cause it refers to items. "Watashi mou toshi desu" would be correct if you're trying to say you're old already... Or something. I get confused with the "stills" and "alreadys."
Yeah, knew the furui was for soylent green, not people.
Mou is generally "more" or "already," mada is "yet" or "still." I think. My understanding of the phrase is that it has the sense of "getting old" rather than "gotten old," but I don't really know kuso.
mou osoi desu -- it's already late mou hitori -- another person, the other person (depending on context) mada samuku arimasen -- it's not cold yet mada desu -- not yet (a little irregular, I'd think this would be mada arimasen or something, but whatever)
But saying you're old is one of those weird idiomatic things that Japanese seems to use for everything worth saying, apparently.