Are stat boosters better to use as early as possible or later when a Pokemon..

It doesnt matter as long as it's before level 100 if you want the evs.

You can use it at any level I believe. It is better to pump them in ASAP so you have knowlege of all the EVs are going. At later levels, EVs build up, so it will be tough to fix the EV inbalance when you don't know how much EVs your Pokemon has. Every Pokemon gives off EVs. What you battle depends on what EVs you get. Machop and Kriketune give off attack EVs and Geodude and Onix would give out ones for defense. Gastly and Haunter are good for Special Attack while Tentacool and Mantyke are good for Special Defense. Speed EVs are given out by Pokemon like Zubat and Starly. Gastrodon and Chansey give out EVs for HP. How evolved they are also factor into EVs.

Ex. Machop gives off 1 Ev in attack, but a Machoke will give 2. Geodude will give off a Defense EV, but a Graveler will give out double the value in defense.

Holding a Macho Brace will reduce you Pokemon's speed, but it doubles EV gains. Giving Vitamins ASAP is the best way of knowing where your EVs are going. You can oly use Vitamins until the EV total for that stat goes to 100 or beyond. You can only have 255 EVs in a stat and 510 EVs total. 4 EVs=1 stat point. 255 doesn't evenly divide by 4, so that is 3 points wasted. Instead, people put in 252 in the two most important stats for their role (sometimes they will split one and equalize two stats) and 6 in something like Hp. This way only two points are wasted. EV training takes awhile, but it is a critical factor in how Powerful your Pokemon can be.

This is what I understand of EVs. I hope this clarifies your question.