Well, you can look at age timeline:

Start school at 7;

Start university at 19;

Start PhD at 25 [4-year bachelors + 2-year masters] if you can do that immediately after masters, or 26;

Finish PhD at 28, or 29 (if it's a 3-year PhD, otherwise older);

Finish first post-doc at 31, second one at 33/34.

By mid-30s you are just starting your tenure-track (assuming you get some semi-permanent position at a university or research institute)

I'm currently doing a PhD [quantum chemistry] and this is basically my expectation for the future.