Is it possible for a 13 year old to already have endometriosis?

Possible. Millions of women have endometriosis, many of them teenagers. Sometimes it is hard to diagnose in a young woman, after a girl first gets her period it is assumed maybe her painful periods are cramps.But when there is long standing pain around the cycle and no other reason can be discovered it may be suspected. The only way to diagnose accurately is by having a laparoscopic procedure.
Yes! While most cases of endometriosis occur in women 25 to 35 years of age, there have been reports of patients as young as 11 years old. Studies suggest it is more common in tall, thin, low bmi women who are caucasian, rather than african american or asian.