re: How old should a athlete be to be considered NFL ready?

Being 24 doesn't make them better players or more talented plus those kids take 2 years away from football meaning no development in that period. A 275 pound OL with basic hs coaching is at a considerable disadvantage going against 30 year olds with over a decade of top notch coaching and development. It's an apples to oranges comparison.
Better comparison using your argument are the hs kids "held back" for athletic purposes.