Buying a bike: Older, used high-quality components or new average-quality components? I realize that fit and personal comfort, etc. are most important, but, I need some advice.
I just got into duathlons and triathlons this year, and started with a very low end bike. I'm now looking to upgrade, but not trying to go crazy.

My question is regarding good versus average components. Are older great components better than new average components?
For an average rider, with some racing goals, etc, does new outweigh better, or does quality always outweigh age?

None of the items are bad, just older or better. 

I'm specifically comparing a used 2007 bike with a Shimano 105 groupset and a 2016 bike with a Tiagra groupset.
Trying to remain general - and assuming both bikes are a good fit for you, I'd base the decision on Warranty.

The older bike is a decade out of date and will have a decade of use on it... but that could be anything from 0 km up to 50,000 km of travel.  So it should be in the "couple of hundred dollars" range at most.

The new bike will have a store warranty.  If you feel you will need that support, then a new bike is a reasonable idea.  But the new bike will cost you five to ten times as much as the used bike.