A screen shot of applications for Apple Inc.'s iPad and iPhone is displayed for a photograph in San Francisco, California, U.S., on Tuesday, July 19, 2011. (David Paul Morris/BLOOMBERG)

Vine, the six-second video app owned by Twitter, has updated its rating in Apple’s App Store to warn users that it’s user-generate content may not be suitable for children.

The video-sharing service allots users six seconds to craft a creative video; some users have a more adult interpretation of that edict. The rating follows a controversy Vine faced soon after its launch last month, when a pornographic video was accidentally featured as an “editors’ pick.”

So the app now carries a 17+ rating on the marketplace, citing the possibility that users may come across videos with references to drug use, sex, violence, crude humor, bad language or gambling.

Vine videos are public and the app’s main screen shows users trending videos as well as those posted by friends. Because the videos play automatically, it can be difficult to assess its content before it begins playing — and a moments’ hesitation really matters when each video is, at most, six seconds. The service does allow users to flag videos they see in the app as inappropriate, and have screened out videos tagged with keywords indicating adult content.

Almost every photo and video service that depends on its users to generate content faces the dilemma of screening out content that younger users and others may find objectionable. In most cases, filters and user reports keep the most violent or pornographic content off of services’ main pages.

For example, YouTube, which has a 12+ rating on the app store, faces many of the same issues that Vine does. It has an elaborate reporting and scanning system that moves quickly to remove and age-restrict inappropriate videos. YouTube has a Safety Mode and says it’s constantly reviewing flagged videos.

But even with its precautions, the Google-owned video service still has some problems with age-appropriate content — Kaspersky Labs recently warned that children are always just a few clicks away from adult content at any time on the site.

Even with the warnings in place, it’s always a good practice for parents to check in on young Internet users and, when possible, watch videos with them to steer clear of videos that have made it past screeners.

Related stories:

Vine’s porn problem won’t hurt Twitter, analyst says

Kids are three clicks away from adult content on YouTube, study says

Twitter said to mull stricter on-site security

Instagram expands Web site

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