Recently 4 month old yorkie has started humping and lifting his leg and peeing pretty much anywhere.

He's 4 months tomorrow.
It's too soon to have him neutered right? And would that even help now that these behaviors have started?
Ok and this is random...is there anyway to tell a dog's true age? This stuff along with some other things is making me doubt his real age. I just have an icky feeling about the place I got him and I'm wondering if they lied to make him seem more desirable.