The first part is that you may be a "lucky stiff" on multiple fronts. Puberty winds down at different ages and some men go on at the tail end of it for years after others. The constant erections over nothing are a just maybe indicator that you have a little more size still heading your way. These constant erections are stretching erections and are a marker that you may still be growing.
The other thing that causes this to a degree is simply lack of a good sexual outlet. When I was your age, it took a great deal to really satisfy me. It did happen, but I had to really go at it until either my sex partner or I were totally at a state of near collapse from going at it.
I am 55 now and I remember those days well!
Kick back and enjoy the fun you're having!