At what level does Tangela evolve?

The earliest level a Tangela can evolve is Level 38. Tangela evolves once it has learned the attack Ancient Power and levels up while having this ability.

Once a Tangela evolves, it becomes Tangrowth. It still retains its grass type in this evolution. It also gains the ability to learn a few attacks that Tangela cannot, such as "block," which prevents an enemy Pokémon from escaping.

All of Tangela's passive abilities remain throughout the evolution, and most of its base stats change very little, except base Hit Points, which increase by 35 points. Note that this evolution is only available beginning with Generation IV Pokémon games, because before that version, Tangela did not have an evolution.