I'm not sure if it's late game either by champion level or by the game time.
People say "early phase you won't do damage, but you do at mid game"
Anyway to refer when do these phases begin?
Upvoted both other answers because they are close but I think their answers are still slightly off.
Each player sticks to their associated role/lane.
First tower or 2 goes down. People begin regularly leaving lanes in order to try helping push other objectives and grouping up.
1 or more players have 3-4 full items. (Sometimes you hear someone say something like Jax got to his late game really fast after being fed or they never got a chance to reach late game after being shut down so hard.)
These answers are pretty similar to the others, but I think it gives a little bit more important info about what each part of the game is.
It varies from game to game, but in 5v5 SR matches, it typically is as follows:
This is the start of the game. Each player is playing their role (jungle, adc, support, etc.)
Mid game typically begins when dragon is regularly being contested.
Late game starts when 5v5 team fights (or 4v4+1 / split push) becomes regular and when baron is regularly being contested (or defended).