The next three ranks after grown-up are unlocked when you get to the top 10%, 4%, and 1% in the UK in terms of points. I’m not sure what that equates to in terms of points these days.
More info here:
https://www.waze.com/discuss/t/your-rank-and-points/377943#p-2277044-waze-levels-waze-points-levels-13