Not all educational apps are created equal. Some are more fun than others. Some are more pedagogically sound than others. And some are better for certain age groups than others.
In the App Store, it is difficult to find out which apps are best for a particular age group, like, say, middle school students. Fortunately, technologists like South Carolina math and engineering teacher Chris Beyerle actively curate collections of apps. Here is his collection of math apps that are appropriate for middle school students. They all fit within the Common Core Standards of Mathematical Practices as well.