You can max out a Pokemon's EVs at ANY level you wish. You will get the same boosts from Super-Training at Lv1 as you would at Lv99.
Because you're new to 6th Gen I will remind you or tell you if you haven't discovered or heard of the new EV technique which is so easy compared to Super-Training. Although you can only do it a little further in the game but it's good to keep in mind as you can fully EV train multiple Pokemon at a time within 5-6 hoard battles. The method is here
And yes you can reset a Pokemon's EVs, however it erases every single EV you've obtained. You can find out how to do that here. You basically just keep hitting the blank punching bag on the Super-Training screen on the bottom of your DS and it will randomly give you bags and one of them will eventually be a 'Reset Bag' have a Pokemon beat that and it will have 0 EVs. The link will demonstrate and explain it a lot better than doing it verbally.
... Happy Christmas!