I do tend to imagine Civ7 ages based on political systems / social structures more than closing them strictly to determinate years. Of course, this still won't work perfectly for everyone, but at least helps me make sense in my head.
Antiquity Age is the age of empires based on a "tribal" entity where a single family or city rules over conquered territory or dominates a league of tribes. Final crisis / collapse is due to unability to handle foreign influence (hunnic/ mongol invasions), impossibility to keep the league of cities toguether (alexander's succesor states, mayan collapse) and emergence of religions as a separate entity from state, challenging god-king status.
The european "middle" ages, as its name implies, is the transition phase (early middle-ages / dark ages being antiquity crisis itself or its aftermath, and low middle ages actually included in exploration age).
Exploration Age empires will be caracterized by the handling of religion as a different "power" to deal with (expect here the full-fledged religion system with missionaries and inquisitions) instead of something given, as well as more complex commerce and society systems (feudal relationships were the strenght of the suzerain matters more than the cultural-familiar alignement later evolved to colonial systems). Nation-state identity is being formed here as a counterbalance to the tensions of feudal system, however in the end will contribute to crisis. In part as religion transition from leader-associated to a separate system may help ending antiquity, the emergence of the "rich" (burgesy) as a power, the challenges to the structure of the world (and to religion) brought by scientific/cultural progress, and the "national" identitiy being moved from the ruling dinasties to the actual people of a region brings in a new crisis in form of revolutionary movements (either independence of colonial elites from metropolis or republican/liberal movements against absolute monarchy).
This leads to the Modern age, fueled by nation-state sentiments and burgesy enterpreneurship, and directed either by the Monarchs that were able to change focus and channel them, or the new Governments / Revolutionaries-turned-
DictatorsLeaders that overturned the Old Regime. Enphasis here is in the progress (scientific and economic) of the identified cultural nation (however loose or exact it can be, propaganda takes charge of identifying who is us and who is not) and a second "colonial" wave more based in cultural/commercial dominance than actual ownership of the lands.
Ultimiately this Modern age structure will lead to a crisis with the clash of powers that leads to WWI, WWII and cold war and the menace of mutual assured destruction due to the evolution of weapons and tactics.
This potential crisis is what leads me to consider the current "Globalization Age" is different for the "Modern Age" depicted in the game and may deserve its own separate period in an expansion (extending into the future). Howerver, as
@Eagle Pursuit or
@UWHabs said, some issues may appear in the risky choice of civilisations in the current scenario or interpretartion of future evoluitons of the world (yet, this may be one of the reasons to save it for a DLC).
The possibility of an initial pre-Antiuity Age is algo possibilty, with the Late Harappan - Aryan Migration - Bronze Age collapse framework as the crisis marking the difference between the "early Antiquity" and the "classical Antiquity" periods. However, this dives into a darker part of history were I do not know if we have enough established knovledge (or trivia) regarding pre-crisis and post-crisis values and societies to be able to differentiate the ages in a sensible way. Of course, you could go as
@Professor Phobo said and assing that "Bronze Age" the fantasy elements present in Epics, but I'm not sure that would be the preferred way for most here.