[Aiwendil]

Just a few minor quibbles:

First - - creation of Ea - - - - Morgoth cast to the void

There seems to be some confusion concerning the "First Age". I was always under the impression that the "ages" that we are discussing are really "ages of the Sun". There were ages before the first age (the ages of the lamps, of darkness, and of the Trees). I would say then that the First Age is from the first rising of the Sun to the casting of Morgoth into the void.

Second - creation of Numenor - destruction of Numenor

Actually, Numenor was created a few years after the beginning of the age and was destroyed prior to its end. I would say rather from the expulsion of Morgoth to the Last Alliance.

Third - - institution of Gondor - destruction of the Ring

From the Last Alliance to the destruction of the Ring.

I agree that in a Christian mythology like Tolkien's, the incarnation of Christ would certainly mark the beginning of an age. Perhaps also the covenant with the Jews?

But I think it is difficult to pick appropriate starting/ending places. Even in Middle-earth, ages didn't always end where you'd expect. The fall of Numenor seems like a much more apt place to end the Second Age than the Last Alliance.