I went through a similar process as yourself not so long ago. I recently purchased a Netgear NAS to provide consolidated, instant access to both critical (documents) and non-critical (media) files. then I also wanted to update my backup solution to backup the data on the NAS.
Firstly, regarding the drive age question:
I agree with the not-broke sentiment above, however I suggest a couple of things to do to reduce your risk of sudden catastrophic failure.
1) pre-clear the disk. This basically just smashes the drive with I/O for an extended period and uncovers bad sectors in the process. (http://lime-technology.com/forum/index.php?topic=2817.0)
2) run something like 'stablebit scanner' on drives that already have data to 1) review SMART data in an understandable way and 2) find bad sectors.
I ended up having to bin 3 drives after these steps cause they weren't up to snuff
Ultimately, I went with FlexRAID for my backup solution for a few reasons which I think may answer your question:
1) It runs on windows and has a GUI
2) it has a number of smart data protection technologies built in (http://snapraid.sourceforge.net/compare.html)
3) It removes some of the drawbacks of keeping backups on proprietary RAID systems, as discussed below;
4) You can use drives that you have lying around to create the backup array and use all their capacity towards the array (either storage or parity protection)
5) It has additional levels of "protection" that are nice compared to something like RAID 5:
you can un-fail drives if they get dropped from the array but actually have nothing wrong with them
if your array screws up for some reason, you can access the remaining files on the drives using windows, without fancy recovery software
Ditto the above if the controller/mobo dies. If this happens with a hardware solution you need to find the exact same controller with exact same firmware if you need to recover the array
I personally consider these things combined as more than 2 levels of protection for my data: 1) Netgear NAS 2)FlexRAID parity to recover 1 failed drive 3) recovery of remaining data in a standard windows environment if it all really hits the fan.
Hope this provides some insight for you.