I tend to be both of the receiving and the wrong guesses end. I really am 16 (well, 17 in ten days, but the same thing), but people tend to guess that I'm in my twenties based on physical appearance (which actually makes it fun when they offer me alcoholic beverages in restaurants; not that I ever take them up on it, of course, but my mother's expression never gets old); likewise, my mother, who's in her forties, is guessed to be in her thirties.
Personally, I simply suck at guessing ages. A boy I thought was five turned out to be ten; a girl I thought might be eight was actually thirteen. I'll guess older too; the most embarrassing example of such was when I told someone that she didn't look older than 45 (and I was rounding down; she had made a remark about looking ancient) and found out that she was actually 39. I've gotten to the point where I just guess on categories; baby/toddler, young-young, youngish, teen, young adult, 20s-40s adult, middle age, old, really old.
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"Nothing worth having is easy."
Three years!