Superman was well into his late 20s if not 30s when Dick became Nightwing, well after the JL was founded. The New 52 had him be a recent college grad when the JL was formed, but the overall main continuity has to function that way as well since we use Dick's growth as a measuring stick for how old everyone else must be. He has to be in his early 20s for it to work, since DC is only now vaguely indicating he *might* be in his early 40s well after he's married and has Jon, and Dick has to be in his late 20s.
He was in the 22-24 range, which isn't a huge gap from 19. We also know less of the circumstances of the JL in the future, maybe they don't have many options either? Are the Teen Titans ever still a thing? And when one of the alternatives is Damian, a 19 year old isn't a stretch.