In the new episodes, they're rebuilding the Air Nation. I doubt they're being taught as strictly as the Eastern Air Temple is now. Furthermore, Jinora is the one whom got her tattoos, and she is the grand daughter of Aang. 

It's not a good idea to question the precedent set by the Mod. Even if it happened a handful of times, it's not enough to justify commonplace exceptions. That's the reason why the mastery tattoos that Jinora received aren't arguably as difficult to get as the ones Aang received. Different cultures at different times. In an era of rebuilding, the rules are most likely more lax than they would be in the stoic era of attempted enlightenment.