It Evolved prior to the episode Staging a hero's Welcome!, When it was traveling with May in Jonto!
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You do not need to evolve Squirtle using an Evolution Stone. Squirtle evolves into Wartortle at level 16.
wartortle
Level 16
No, you cannot evolve Squirtle with a Water Stone.
Squirtle evolves at lv 16 into wartortle and at lv 36 at Blastoise .