Pikachu does not actually have a set level for evolving into Raichu. Pikachu evolves when the player uses a Thunder Stone on it. This can be done at any level.
Thunder Stones are semi-rare stones that come in all elements and are used to evolve specific Pokémon. A Thunder Stone can be found in several places throughout all of the "Pokémon" games. However, once Pikachu evolves into Raichu, it does not learn any new abilities. Most trainers recommend waiting to use the Thunder Stone until Pikachu has reached level 50 and has learned its final move, Thunder. Raichu can be taught moves through other means, such as through HMs and TMs, but these can be rare to obtain and may be rather expensive.