The rule is not age discrimination, like vinny mentioned some 20year olds are eligible some aren't. The functional part of the rule as it applies to %98 of the draftees and what I think the court was referring to could be paraphrased as: you need to have played/learned/studied college football for three years.
I don't see that being any different then a job posting that reads 'applicants must have 3 years of experience to be eligible for this high paying job.' Using the years out of high school stat just allows for the rare exceptions that come up. These guys don't have a right to play in the NFL, they do have the right to apply and be fairly considered--once they meet the necessary requirements.