That happens only if you are on the second highest level on your server or even third highest. I'm only level 17 and the highest level on my server is 22 (there are only 2 players at this level) playing in a league with players from level 21 and 20. It will take a while (probably at least 6-7 months) to join them
That is not correct. I played in higher level leagues when I was levels 6 & 7. At the time, the highest level in my server (that I knew of), was level 21.