Horses do mature at 5 years old, but that's like saying it's okay when teenagers do things that are just down right DUMB when they are 15 because their reasoning part of their brain isn't there yet. Yes, we can't expect them to be perfect, but we have to shape them- no just wait until one day they wake up and say, "Well, I'm 5 now so I have become a mature horse!"
I think that as horse owners we need to find what works for us, and the horse, as our punishment. It's great when you have a horse that aims to please and responds to simple things like stopping your foot and body language... that's what we all WANT horses to do. A strong bond and respect is clear in a relationship where a horse will respond to that, because that shows they respect you but do not fear you.
However, plenty of horses DON'T have that attitude. As long as you're not HARMING the horse, I think getting after them with more strength is important. It shouldn't be your first response, always aim for the body language, voice cues, etc... but if need be, it doesn't hurt a horse to be reminded. It's like a bite from an elder horse, the last resort but sometimes it's in line.
My horse is 11, and he's a total BOOGER when he hasn't been worked with in long periods of time. He's bitten my leg, pranced around and not paid ANY attention on the lead rope, etc. They all go through periods of time where they test you, but you have to remember a good relationship with a horse doesn't come from dominance over them but rather respect coming from both sides. If you work with him a lot, which it sounds like you do, he'll get through it fine. I'm sorry he's being a brat, but one day all of your work will pay off and he'll have excellent ground manners and you will be able to say you did it all on your own- and it'll be based on respect of each other!
Proud owner of 13yr old Paint Gelding, Bali! <3