I went to an amusement park and one of the street entertainers guessed my age. How did they do that?

An experienced Guesser will count the lines on the forehead for the Decades and  the crow's feet and other lines for years.   This guy is about 32.  Practice by studying the face and lines of a few people you know and see if it works.  Certain ethnic groups and individuals will differ (a Tibetan monk for e.g.)but for the most part its good.  There's no reason he's sticking his tongue out; I just drew it like that.  .