A calculator is a good tool if you don't want to do math: https://calc.pokemonshowdown.com/
If you get a level 1 Tyrogue with 0 IVs and EVs and a neutral nature, then its HP is 11, and all other stats are 5. If you put 252 EVs into any stat, then it increases by 1. So EVs can take effect at any level.
However, a level 1 Tyrogue's stat needs 120 EVs to increase by 1, and any more EVs have no effect. At level 100, a stat will keep increasing until it has 252 EVs. If you want a fast attacking level 1 Tyrogue, then the best spread is probably 120 Atk/120 Def/120 SpD/120 Spe, while the best spread for a fast attacking level 100 Tyrogue is probably 252 Atk/4 Def/252 Spe. So you'll want different EV spreads for different levels.