Well....no, it isn't. That's kinda the point. The NFL has an antitrust exemption and currently has (I believe exclusive) contracts with every major broadcast network in the country protected by that exemption. They limit the participation of their member-teams through the NFLPA's requirement that you be three years out of high school. That's a restriction on a capitalist market.
Further, the no football is allowed to be broadcast on television on Fridays or Saturdays in the Fall within a certain radius of any high school or college game being played (this is why the NFL playoffs don't play on Saturday until AFTER the college season is over). That's part of the NFL's antitrust compromise that helps the NCAA and high school football, but cripples any attempt at competiton. Again, restriction on a capitalist market.
Even further, courts have pretty consistently ruled that NCAA players are ineligible to form a union at both public and private colleges, meaning whatever you think of collective bargaining or Unions in general, they are not afforded the same privileges that a professional ballplayer is allowed.
So yes, technically, anyone can start a league that would theoretically allow under 21 players. That's what we're seeing Vince McMahon try right now with the revival of the XFL in the spring. But in addition to the MASSIVE barriers to entry that any new league faces, the deck is already stacked against them from a regulatory standpoint.