Originally Posted by
Uzor
This is somewhat wrong. Egg pokémon caps at 20. Wild pokémon caps at 30.
The reason you shouldn't power up until lvl 20 is as he mentioned though -- because until lvl 20 you'll soar through levels like no one's business. You can get 6 lvls from a single lucky egg. After lvl 20, exp needed to get to next lvl goes up by A LOT. Right now at lvl 24, I need 150000 exp to get to lvl 25. At 15 it's, what? 15000? Before 20 you'll find a stronger pokémon before you even have time to max a pokémon out. Past lvl 20 you'll get levels much slower, so you have time to cap a pokémon out every time you get a new lvl if you find one with good IV.
I mean, sure, if you find a pokémon with good IV at lvl 10, by all means, keep it if you think you'll use it. But you'll spend a lot of candies and dust in vain since odds are you'll find a better one or equal at higher lvl that is already close to its CP cap. This is especially true for something like a Dragonite, Snorlax, or any other pokémon that's rare and hard to get candies for. It's not as much of an issue for an Eevee.