Questions about adolescence and barking!

Hi guys, my pup Prince is about 5 and a half months old, and we are beginning to hit the dreaded teenage phase, hes pushing his limits on what he can do, and being quite cheeky. I just wanted a bit of insight into what else we can expect from this period, how long will it last? What are your experiences? Any tips?

Also, i assume it is related as it started at the same time, but every time we go into the garden ( only the garden ) he will bark at seemingly nothing. Ive heard of fear barking and territorial barking, is this just a side effect of growing up?

Thanks for any feedback!