IMPORTANT:

Tony, Peter, and the Guardians stopped Thanos when they were on Titan during Infinity War. Basically, Peter Quill kept it together long enough for Peter to take the gauntlet, and then they all killed Thanos. The purple guy never snapped and all is jiggy.

Also:

This is three years after Infinity War (but since they won, no one dusted). Peter graduated high school a year early and is now a sophomore in college at NYU and is 19 years old.