This is just a theory, but it seems to be case that some starter evolutions are based upon the gym leaders.
Example: Chimchar evolves into Monferno at level 14, Piplup evolves at level 16 and Turtwig at level 18 because the first gym is Rock.
Evolving Chimchar into Monferno is the only way you take on Roark with your starter without being trashed. However, Piplup has super effective moves against Rock & Ground, whereas Turtwig has resistance to Ground as well as super effective moves. It also has the highest defences, meaning it can tank Roark much better than the other two.
As for Venusaur, look at the gym leaders after level 32; Koga/Janine, Sabrina and Blaine. Venusaur is weak to all of them, so maybe Game Freak just decided that since you won't be using it against those gyms, might as well give it an early evolution boost so training it after not using it for 3 gyms won't be that hard.
I don't think the theory works for all generations though. E.g. all Hoenn starters evolve at level 16, despite the first gym being Rock. However, you could say that because Roxanne's Pokemon are at level 14-15 and it takes a long time to get to her, your starter should've evolved by then anyway so there's no need for them to have different evolution stages.