Good point, Technical – and if it’s not currently possible, it would make a nice addition to the “wish list”.
Ad-Aware (the relatively new SE, that is) added a feature that warns if you start a scan but your defs are more than 14 days old. And I seem to remember that either in A-A’s own setup, or in its “Tweak” plug-in, there’s an option to change the age that’s considered too old.