Generally, we conventionally agreed the start and the end of the historical ages were determinated by decisive events.
End of Ancient Age / start of Middle Ages: End of Western Roman Empire (4th September 476);
End of Middle Ages / start of Modern Age: Discovery of America ( 12th October 1492);
End of Modern Age / start of Contemporary Age: Congress of Wien ( 1st November 1814) ( However, personally i choose as start date 14th July 1789, the start of French Revolution).
However, if one of those events will not happened, how will be affect the lenght, or the denominations of the various ages? For example, a missed discovery of America could determinate a more long Middle Ages? The historians will adopted other dates? The same ages will conserved the same terms ( example, a surviving WRE could however determinate the concept of "Middle Ages" ) ? Other ages will be created?