In William Shakespeare's Romeo and Juliet, the exact age of Romeo is never explicitly stated.
Juliet's age is 13, and she is almost about to turn 14. Based on puberty (girls hit puberty around 11-13, while it takes a few years longer for boys), we can definitely assume that Romeo is not younger than Juliet, but older.
How much older though?
He could be anywhere from 13 / 14 to his early twenties. We know he is "young" because several characters, including Friar Lawrence and the Nurse, state this adjective about him. Although "young" is relative, we can assume that in this time period and culture, "young" has to be younger than early twenties.