Most starter Pokemon first evolve at level 16, and then again at level 36. It would surprise me if this wasn't intentionally done for balancing reasons. Not too early that it gives you an excessive power boost, but not too late that you feel underpowered before it evolves. There are are, however, several Pokemon that break this norm by a few levels. Ivysaur, for example, evolves into Venusaur four levels before its Water and Fire type counterparts evolve into their final stages. All of the Unova starter Pokemon evolve into their second stages on level 17, one level later than what seems to be the average. The same applies to the Alola starters, though they also evolve into their third forms at level 34, two levels earlier than average.
So, why break this norm? Again, it seems pretty intentional that most of them evolve at level 16 and 36. Are the starters that evolve earlier known for being a little harder to use early game? I believe I remember some mention of choosing Bulbasaur making early game Gen 1 a bit more difficult, so could this perhaps be an attempt at balancing this aspect? I don't expect there to be any official mention of this phenomenon, just to be clear, though I'm curious how this aligns with things like type matchups of gyms and common types throughout the different regions.
