At what level does Poliwhirl evolve in Pokemon?

Poliwhirl evolves into poliwrath when you use a Water Stone on it. It can also evolve into politoed if you give it a King's Rock to hold and trade it. It does not evolve by level.

Poliwhirl evolves into poliwrath when you use a Water Stone on it. It can also evolve into a politoed if you trade it while it's holding a King's Rock.
Poliwhirl does not evolve at a level. Poliwhirl can evolve into poliwrath with a water stone, or poliwhirl can evolve into politoed when it is traded while holding a kings rock.

Hope I helped!