When do gsd's come into heat for the 1st time?

I have a 6-7 mth female gsd, she has been crate trained, my cousin needs me to watch his lab male puppy (2 mths) while hes on vacation. i dont keep him and her in same crate but when they play she kinda forces him to lick u know what, and wants him to stay under her..lol.. she then starts humpin around uncontrollably for a few secs... does she really know whats goin on at this age? should i let this happen? should i keep them seperate? any suggestions?