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

Fournette could have played in the NFL right out of HS, and he plays a position that takes the most hits. So I don't think there should be any age limit. Really depends on each individual athlete. Skill guys like RBs, WRs, Kickers, Punters could play right out of HS. They likely aren't developing much anyway in college either physically or mentally.

All the other positions take lot of development time so doubtful they could make that jump or NFL teams would want to waste those picks knowing they wouldn't be ready for 2-3 years.