By the time most uncircumcised boys are 10 years old, they can pull the foreskin back from the head of their penis. For some, it may not retract completely until they are about 17 years old. When this happens, it’s called phimosis.
Boys who have it are born with phimosis, and it can last through puberty. With time, the skin retracts off the head of the penis naturally. You need treatment only if it happens after your foreskin has become fully retractable. Or if, as a child, the head of the penis has redness, pain, or swelling.
When the foreskin gets stuck behind the head of the penis, in paraphimosis, circulation can get cut off. Men or boys of any age need to take this seriously and see a doctor or go to the hospital immediately.