It has always been born out in the show. I was actually going to bring up the Kakistos point as well. Then there is the master, who spews black stuff for 2 minutes while dying.
We also have examples of (age = strength) from the Angel series. We have Darla lifting Angel right off of his feet with one hand around his neck, and holding him at arms length (she is after all 150 years his senior, in earth years anyway). At the same time, Drusilla seems considerably weaker than that, but still strong in her own right.
We also get to see how even Drusilla can cow younger vampires with her stength. She's hardly a warrior-princess, so they couldn't have been cowed by her kung-fu skills. No, I think they were in fear of her terrible strength.
We have other examples, as the two strongest of the master's original servants (The harbringer and Darla) were vampires of great age. We have Dracula, who is able to do all sorts of unusual things, and is also of (relatively) great age.
On the other hand, we then have Spike. He's of significant age, yes, but he also possesses a romantic fire, that when ignited, makes him a more capable fighter than he really should be. Spike without that fire is good for comic relief and constant comedic failure. Spike with that flame ignited and burning strong is a force to be reckoned with.
So I guess it's safe to say that Age isn't everything. It does however, mean a great deal.