Of the Nexus One (2010-01), Nexus S (2010-12), and Galaxy Nexus (2011-11), which can run Android 4.4 (KitKat)? Nexus One is stuck on 2.3 (Gingerbread), Nexus S on 4.1 (Jelly Bean), and Galaxy Nexus on 4.2 (Jelly Bean 2) or 4.3 (Jelly Bean 3) depending on revision.
True enough, however, if I REALLY wanted my old Android device (Nexus S for example) to run a newer version of Android, it COULD be done. Root the phone and load Cyanogen or something. With an iPad, from my knowledge, you're pretty much SOL.