Frankly that's just not the niche NAHL is going for. There are already 20 year olds across the entire Junior Hockey landscape. This league is essentially saying, "we'll give you a place to play, so come here". The tradeoff here is that a whole bunch of players are obviously willing to do so and delay school another year in order to improve their College Hockey prospects. And then with such a system in place, college teams, such as lower NCAA D1 and D3 teams would rather have older players because they aren't chasing high-end ceilings or future NHL players, but players they can plug and play into their system for four years, so may as well get the guys who are the most physically mature already.
It's all pretty arbitrary what number is chosen, but junior hockey as a whole has chosen Age 20 as the last for junior hockey. A Tier II league NAHL isn't going to get a bunch of 17 year olds because the highest end ones will be playing in other junior leagues like one of the CHL, USHL or BCHL, and the next batch will likely not be packing their bags and doing the whole Junior Hockey thing while they're still in high school. By 20, they are an adult, out of high school and ready to do that before they enter College.