Nooope. I'm currently twenty-five and no one ever correctly guesses my age. I look much younger than I am, due to my short stature and inability to grow facial hair. It's irksome, because some people act particularly condescending to me. I try to dress maturely, but it doesn't really work because I am so small. I end up looking more awkward than not, I think. Often, I am glared at when I go to bars and I am carded every time and once I was accused of having a fake ID. It's also brought me some trouble when landing a job, I've noticed. Something about me looks inexperienced, or they assume this based on my appearance, despite my work in wedding catering.
On the flip side, it is occasionally advantageous. I've used it before to win a prize for a "guess your age" sort of thing before. The guesser was horribly off, so I ended up winning the grand prize for a dear friend of mine: a giant Croagunk doll, which was his favorite Pokemon.