This isn't a rule anymore. You can go all the way to 40, but you'd have to go through WLDQ rules/requirements if you wanted to go WL at some point, no earlier than level 21.
Slight correction to this...
One can take a WLDQ starting at level 20 and use it to crest the somewhat significant jump in XP needed to go from level 20 to level 21 (something like 3million, iirc). If you pass the WLDQ while still 20th level, you get an automatic bump to level 21.
@silverblades, could you post a link to where you found this rule still in existance? It clearly got missed when we revised this rule (quite a long time ago), so we'd like to clean it up to avoid this sort of confusion in the future.