This is just my own personal experience:
I have over 300 different concentrates in my current inventory. Of those, some 70-80 of them are anywhere from 2-3 years old. I should also note that most of these 70(ish) are 8-10mL sample-size, and are in glass vials with poly-cone caps. (Important; because this is near optimal storage conditions. Larger containers, plastic containers, poorly sealed caps may have had entirely different results). They have all been stored on a shelf in a cool dark linen closet, in the center of the house, since they were purchased.
Of those 70+ concentrates, I have some that I detect no noticeable difference (of course this could be because I don't use them often; and I don't have a good recollection of how they tasted originally
), others that seem pretty much the same but weaker (for some the weakness is minor; while others are significant), while others now taste "different" (meaning that originally one part of the flavor was prominent; and now something else has taken over center-stage).
Flavor concentrates, as long as you are buying them from any of the established "major" manufacturers (e.g. FA, CAP, TFA, INW, HS, SC, FLV, etc.), do not "spoil" (as in; mold, rot, decay, putrefy). They only need to be tossed once they become no longer useful/enjoyable
to you.
Also, regarding crystallization (particles forming in the concentrate); I have had this happen on occasion, and when it does, it is no big deal. I just shake vigorously, or give it 5-10 minutes in the ultrasonic in lukewarm water, and it remains perfectly usable. I
do use heat
very sparingly; as I am always paranoid about needlessly driving off the more volatile components.
Again; this is only
my own experience; others may share entirely different experiences, and I would encourage you to consider them just as valid.
) 
