What’s the issue here? It doesn’t take long to get to level 20, and then you have access forever. Some people just have to whinge.
They won't give you extended targets until you have proven you know how to, well, you know... keel stuff.
I would imagine its lvl 20 so that new folks can acclimate to the game, get to 20, then learn something new like this window.
It takes a few hours to reach level 20. Most peeps at this point are finishing up their planner gear and getting what they can for epics 1.0 and waiting for Kunark to drop......I dont think not getting Extended target window before 20 is a issue.