Evolve Pansage after it learns Seed Bomb at level 22. It's a good STAB attack that would see you through most of your playthrough. If you don't get Pansage to learn it, then you will have to wait until you reach Mistralton City to re-learn it using a Heart Scale.
Evolving Pansage early will make Clay's Gym easier, so I think that is the absolute latest you should wait. Acrobatics is potentially worth having, but you can also get that move as a TM at around the same time Pansage would reach level 31, so you don't gain much by waiting.