It's been a rough couple of weeks for iOS apps, with both 500px and Vine getting attention for allowing access to adult material. 500px was even pulled for a week before returning with a new 17+ rating — and now it appears the Tumblr iOS app has been updated with a mature rating as well.
Version 3.2.4 of the app, released today, lists "Small bug fixes" as the only change made — but users updating to the new version are finding themselves faced with a dialogue box, forcing them to confirm that they're 17 or over if they wish to update the app. Checking the App Store page for the app itself reveals the 17+ rating. Google has a cached version of the App Store page for version 3.2.3, which lists a rating of 4 and up.
It's not clear if the app was changed by Tumblr as a preventative measure, or if there had been some complaints that drove the decision. Obviously, the Tumblr app, much like 500px — or mobile Safari, for that matter — is able to access a number of blogs that may have NSFW material. We've reached out to Tumblr for comment, and will update you with any further information.