Repairing/ Fixing/ Finding parts for very old Macbooks?
The technician used proper terminology per Apple.  Apple defines a vintage product as one that was discontinued more than 5, but less than 7, years ago.  Apple defines an obsolete product as one discontinued more than 7 years ago.

Your Mac is 3 1/2 years old, thats not yet very old.  With Apple notebooks I typically expect about 6 - 7 years of solid use.  Right now a 7 year old MacBook Pro can still run Lion, which is still fully supported by Apple, thus, in real world terms, making it not yet obsolete, even by Apple’s terms.

When it comes to finding parts for a Mac that fits Apple’s definitions though, that becomes tricky if you insist on “new” parts.  I emphasize new because any of Apple’s service parts for Macs over a year or two old carry a high likelihood of being reconditioned, meaning they were returned to Apple through service channels, diagnosed, fixed, strenuously tested then re-certified for use.  Apple Authorized Service Providers (AASP), including AppleCare &amp; Genius Bar, are able to still order parts for vintage Macs, but that part supply is no longer replenished.

So, say you need a logic board for a vintage MacBook, an AASP can place an order for a logic board and if one is still in stock then the order will be fulfilled.  If none are in stock then the AASPs (non-Apple only) only other option is going with one from a non-Apple source, whether it be a spare they have or from another reseller. 

If you need a logic board for an obsolete MacBook then an AASP (non-Apple only) only has the option of finding one through another reseller or using a used spare they have on hand.  Situations like this are typically the reason a lot of old Macs get retired.

When it comes to sourcing parts on your own there are several reputable vendors that deal in refurbished Apple parts. A few examples:

iFixit - iFixit is also a tremendous resource for walk throughs for the majority of repairs you might encounter with just about any Mac.

PowerBook Medic

MacFixiIt

eBay is also a handy resource although, as always, exercise caution and proper sense when buying from eBay.

As to your specific question, of how difficult would it be to find parts for and repair a 12 year old MacBook... As of right now the oldest MacBook is just shy of 8 years old, so we can't answer your exact question.  However, finding parts for and repairing that 8 year old MacBook is still quite possible in most cases, although many would find it cost prohibitive, meaning they would rather put the $200-400 towards a new or used Mac rather than fixing an 8 year old model.

There ARE 12 year old PowerBook G4s though, and those follow the same idea I laid out in the previous paragraph.  Repair is often possible, although finding logic boards seems to be a bit tougher these days, but it often doesn't make financial sense to repair rather than to replace.

Source - I worked in the Apple break/fix ecosystem for 6 years, for both an AASP and as a Mac Genius.