When you turn 24 your eligiblty clock automaticly starts.
As long as you have not enrolled in a University before then.
So if you are 24 and decide to start University your clock begins and you have the 5 years to compete. So theoretically you can compete up until you are 29.
Well atleast that is how the rule at my university was explained to us. Which made no sense seeing we were already enrolled and our "clocks" had started