At what level is it best to evolve Eevee into jolteon?

as soon as possible

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It's really up to you -- depending on how long you want to deal with Eevee's Normal type, before you evolve it into one of the Eeveelutions. The most powerful move it will learn, as an Eevee, is Take Down, (which has an attack of 90.) That would be level 42. So that's probably the best level to evolve it at.

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