I mean, age, achievement, the knight’s opinion, all of these things played a role. However, there’s one factor that you didn’t mention which was quite significant in determining if and when a squire became a knight: money.
See, a knight was expected to pay for their own arms, armor, horse, etc. and not every squire could afford to do that. So there were quite a few examples of fully-trained, fully-grown, battle-tested men-at-arms who might spend their entire military careers as squires.