Todo Math was created just a few years ago, so it was built using up-to-date iOS software. As a result, Todo Math is only available to be played on iOS 7 or higher.
What does this mean for use on older iPads? Some older iPads can use iOS 7, which means you can still use them to play Todo Math. However, the earliest lines of iPads do not support iOS 7 or higher, which means they won't be able to play Todo Math.
For example, any iPad Mini or iPad air are able to play Todo Math, as can any standard iPad that is generation 4 or higher. iPad 2's and 3's have limited support for iOS 7, so they may or may not be able to support Todo Math. And sadly, the original iPad was never engineered support any operating system above iOS 6, so it won't be able to play Todo Math.
Click here if you'd like to learn more about the different versions of iPads and which iOS versions they support.