Level them up as soon as you can since the moves they learn as Eevee don't benefit them at all!
Just for if you would want more information on their evolution method
You need the following items and locations to evolve tge various eeveelution.
The evolution stones can be exchanged in the battle subway at the cost of 3 BP per stone
(sorry for this error, they can't be purchased in black and white, only in black and white 2. I confused the 2 ›.‹)
-Espeon ( at daytime)
-Umbreon( at night)
-Leafeon (level up near moss rock)
-Glaceon ( level up near ice rock)