Blizzard has made a fairly major change to the way Starter Edition accounts work today (or in the very recent past) in a hotfix.
Reports are coming in from both the official and twinkinfo forums that F2P accounts who have locked their experience gain at levels below 20 are gaining experience. Even if the experience is manually turned off again, they continue to gain experience.
Some forum posters report that GMs are saying that F2P accounts were never supposed to be able to lock their XP, and that this is the intended behavior. We may see some clarification on this in the next few days, but if you have locked a F2P account below level 20, you might want to watch your XP bar very carefully.
This will likely send all 10, 14, and 19 bracket twinks up to level 20, where they will compete in the 20-24 bracket.
I will start revising the F2P guide to reflect this new update over the next few days.
Update: Blue post confirms this was a deliberate change. Vrakthris writes:
Yes, it looks like the option for trial accounts to turn off experience was removed. I don’t believe they were intended to have such an option, so that was fixed.
There have been reports of regular account twinks gaining experience, too. I haven’t been able to duplicate this, but there have been reports in the forums.