My first B-bag purchase was in 2006, and it was a 2005 bag.
I looked at the 06 bags and buying an 05 didnt bother me. If bags had a 'use-by' date or could go off, then I would worry
The store I purchased mine from dont put b-bags on sale EVER. They keep them until sold out.
If they are left with stock from the year before, some buyers will specifically request a bag from the previous year, as they prefer it for whatever reason.
Out of all of the bags I looked at, the 05' leather on my Twiggy was by far the best.
You said that you only noticed when you looked at the tag, true?
I personally wouldnt worry, but it really comes down to the individual. The year shouldnt matter, unless you specifically wanted a 2006 bag.
The leather on my bag was thin and veiny (which I prefer).
I used Lovinmybag products on my bag which gave it some thickness anyway.
My advice: If you love it, keep it!