"I'm pretty sure they are doing 'it' earlier than that. "
An old friend reminded me recently that she and her husband have now been active sex partners since they were 13. That was 60 years ago.
Malinowski's anthropological study of Trobriand Islanders in the early 20th century, "The Sexual Life of Savages", illustrated that was not unusual in a different social system. Surprisingly it did not seem to lead to pregnancies. The tribal elders were amused at Malinowski's ideas on how pregnancy happens. Their belief was that it required the couple to be officially married to produce the necessary fertility. Malinowski had to admit that the practical evidence locally supported their view.
IIRC some studies of monkeys have shown that if they are deprived of "sexual" play at puberty then they fail to mate as adults.