I'll give it a try...
A restriction that all hiring by the company must be thru the union and that employment is based upon seniority. If this restriction is collectively bargained for, any new employees would be at the bottom of the list and if the company is not hiring, they could go years without employment at the company. This is not necessarily based upon age, but it largely affects younger employees.
You can look at the Clarett restriction as one based upon seniority. The NFL will not hire any employee who does not have more than 3 years on-job experience above the level of high school. If this was a restriction by a construction company, this could actually be upheld. Construction, eletrician, longshoremen, etc. unions generally require a certain amount of on-job training (apprenticeship) before the individual is permitted to work for the company on a full time, full benefits basis (learning the trade and how to do things properly). Similarly, the NFL might require a certain time of apprenticeship before Clarett can play. But you might say, well the apprentice still gets paid while Clarett is not. That's not entirely true. He is being paid in the value of his education+monthly stipend+room and board (which is actually far more than any apprentice construction worker could do). Or, he could go play in one of the other leagues (CFL, Arena, etc).
Or let me take the rationale behind the NFL rule. Allowing players to enter the league at the age of 18 would eliminate the viability of college football and ultimately reduce competition in the sports market. The restriction is absolutely necessary to keep college football going. Further, football is a high contact, physical sport where people's lives can often be placed in danger. Allowing 18 year olds, who have not spent the 2 or 3 years in college football developing BOTH their bodies and the knowledge of how to play the game, makes them susceptible to serious injury. A restriction is necessary to let them develop further and avoid injury. This is a health and safety rationale. Preventing injuries is a legitimate reason and the restriction could be upheld. Again, not saying I agree, but that's what the NFL has agrued in the past.
I agree that a restriction based on age is not like the others - that it is difficult to justify. I meant only to give examples where it might be justified. I think the fact that none of the other sports have an age restriction above 18 is going to doom the NFL. They would have to show why the NFL is different from all the rest.