I own a perfectly fine iPad 1 and can't use apps for it anymore because Apple said so. This really irks me, at least let me install the latest usable version.
The iPad is still working nicely, but the behavior by Apple just sucks.
Follow the iOS instructions. You can install older versions of apps directly from the App Store on your iPad by going into the Purchased section. It will prompt you to install the last compatible version.
If you're trying to do this with an app you haven't previously purchased, you'll need to purchase it directly on iTunes, and then it will show up in the Purchased section of the App Store on the device.
The iPad is still working nicely, but the behavior by Apple just sucks.