Sure they can. If there was a version of the app that was submitted that’s compatible with their device, when they try to download it, it will ask them do they want to download “the last compatible version”.
I’ve done that with my first gen iPad as late as last year to download a few streaming apps like Netflix, Hulu, and Plex.
I have the iPhone 3G somewhere in the basement. For years, it has been a paperweight, it couldn't download anything from the App Store, including apps that originally were compatible. It even lost some capabilities it originally had out of the box, for example it could not sync contacts with Google anymore.
The last compatible download feature doesn’t work on my 1st gen iPod touch running iOS 3, but it does work on my iPad running iOS 5. I’m not sure about iOS 4.
I’ve done that with my first gen iPad as late as last year to download a few streaming apps like Netflix, Hulu, and Plex.