Question

Does a Pokemon get much stronger when EV trained before level 100?

Answer 1

No. It is a misconception that there is any optimal time to EV train your Pokemon. You should EV train your Pokemon whenever it is most convenient to you, assuming this is simply training for competitive play.

EVs are just one moving part of a stat calculation formula that is constantly being re-checked as you play the game. You may think of this like the game is 'recalibrating' all the points your Pokemon has accumulated. Because of this, there is no advantage to EV training your Pokemon at a lower (or higher) level.

A Pokemon EV trained at level 1 will have identical stats once it reaches level 100 to the same Pokemon trained for the same EVs at level 100. The only time this isn't true is in old Pokemon games like Diamond and Pearl, where stat recalcuations only occur upon level up: and of course, you cannot level up a level 100 Pokemon.

Comments

One slight advantage of EV training early is that a low level Pokemon is less likely to have gotten some random EVs that you want to reset.

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That's true, though if this is for competitive I definitely agree it's only a slight consideration.