I'm terrible at guessing someone's age, because I've always looked younger than I am. My hair's starting to go a bit more silvery now, but I was carded as recently as two years ago (I'm 45).
Height is not too difficult, because I can generally estimate the difference between my height and someone else's, with one caveat: if someone is outside the height range that's considered "normal", it gets harder to judge the difference. I'm acquainted with a midget, and if he hadn't once actually said that he was 3'4", I would probably guess anywhere between 3' and 4'. Similarly, once someone gets over 6'5", it's tougher to estimate.
For weight, it's not too hard for me to estimate a man's weight, though if someone is more muscular, it tends to throw my estimate further off. I know that muscle is heavier than fat, but it throws me to learn that someone who's four inches shorter than me actually weighs more than I do (especially since I'm a bit heavier than I would like to be).