I always put Green Lantern 76 in a class by itself. Considering the series was cancelled at issue 90 or so (it was later restarted up again), it's harder to see the direct impact that O'Neal and Adams had on the industry. I would like to think that a start of the Age would be something that would last more than a couple of years. It wasn't like all of the comics stated to become more socially questioning and realistic, etc.
Also I would have thought that if Conan #1 was the start of the age, there would have been a lot more successful imitators - most people probably haven't even heard of Arak and Wulf the Barbarian. It's impact lasted only a few years, if that.
Look at what Showcase #4 brought about - the revival of Flash, Green Lantern, etc. plus it's influence on Marvel Comics and their imitation of DC brought about the FF, etc. Pretty earth shattering. I think GS X-Men #1 and the introduction of the New X-Men had that impact on the industry and comics.
I'm okay with there being an age between Gold and Silver when things like EC comics and the horror "craze" occurred. I'm also okay with there being a gap between the end of the Silver Age and the start of the Bronze Age when you had Marvel Team-Up and Defenders coming out. None of those titles though had the same impact that Showcase #4 did (Living Mummy, The Cat, Son of Satan??)
I think someone posted above that there will never be a consensus about what book started the Bronze Age - I would definitely agree with that!