"Regardless of age" is indeterminate ;-) I don't know for sure whether Aperture 3.5.1 could open and upgrade an Aperture 1.0 library. There's probably some timeframe on it, but nobody knows how long.
I'd say that in general you may want to open your old libraries every once in a while and upgrade them. Have a backup of course of all of them before upgrading.
I don't know for sure whether Aperture 3.5.1 could open and upgrade an Aperture 1.0 library.
The Release Notes suggest compatibility backwards, at least back to Aperture 1.5 - see the last paragraph: A note on upgrading your Aperture library
Matte77, I fully agree with William Lloyd - open the Aperture libraries occasionally. Only this way you will notice, if the drive is starting to fail, and if your libraries are still readable. Otherwise you will risk to discover that one of your libraries is no longer readable at the same time as the bakup drive fails.
Also, you might ocnsider to merge your libraries into fewer ones. This way, it would be easier to keep them updated.