this goes for any class, not just ranger. as a lvl 10 blm, i stayed in Maze till 17. from there, Korr Tunnel til 19. I know you can stay there till 20/21 on just worms but the grp i was with had to disband.
there are also crabs in the tunnel that can take you to 21.
now the trick is to find grps there. the advantage of ranged characters is that you dont need a full group. i never had a full group in the maze, was pretty much just blms and whms and maybe 1 meleer to take stone dmg for us (he got free exp so why not hehe)