Vine’s advisory was previously for users aged 12 and up.
In addition to that change, though, Twitter finally added the option to share their videos after they are posted. With Twitter behind the app, it was only natural that users be allowed to share Vine content to Twitter -- and to Facebook, as well.
In addition to those chnages, Vine now also has the option to report or block a user profile. The new functionality adds more filtering to the already existing option to report a specific video.
In the case of Vine's porn problem, it's less of a porn creation problem than it is a porn discovery problem -- the ease at which porn can be found. Despite the fact that Twitter was forced to change the age rating for Vine, one could argue that Apple cut Twitter some slack.
As we noted earlier, Apple and Twitter are partners, in a sense, considering the deep Twitter integration now in iOS. Apple has, in the past, given the boot to other apps for similar or even less flagrant violations of its no-porn (or really, no "too sexy for Steve Jobs") policy.
Apple could easily have kicked Vine out of the App Store, and then allowed it to return after Twitter added the 17+ rating to it. Instead, the app remained in the App Store, although it has seen that new rating added, after all.