[...well, alright, best not to embarrass her by bringing that up...]
It's perfectly fine to evolve an Eevee whenever you want, depending on what moves you want it to learn. If you evolve it early, it will learn more moves of its evolution's type. I'm not exactly sure how the other one evolves, but if it evolves by trading or happiness, you should probably evolve it earlier rather than later-they generally tend to learn similar moves.
As for that Litwick of yours, though... after it evolves for the first time, you will probably want to hold off on evolving it with a Dusk Stone. As a general rule, Pokemon evolved with stones either stop learning moves or learn very few moves upon evolving.
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It's perfectly fine to evolve an Eevee whenever you want, depending on what moves you want it to learn. If you evolve it early, it will learn more moves of its evolution's type. I'm not exactly sure how the other one evolves, but if it evolves by trading or happiness, you should probably evolve it earlier rather than later-they generally tend to learn similar moves.
As for that Litwick of yours, though... after it evolves for the first time, you will probably want to hold off on evolving it with a Dusk Stone. As a general rule, Pokemon evolved with stones either stop learning moves or learn very few moves upon evolving.