[HoneyBadger]

Here's my attempt at explaining what everyone else already did a pretty good job of explaining already, in more or less the same way, but maybe a little different:

Early age usually has the most personal power (from their bodies and minds) per individual, while Middle age has the most individuals, while Late age has more individuals becoming more powerful through their equipment.

It's also sort of a balace between magic being very powerful and stable in Early age, and technology being either very uncertain or weak, to Late age technology being powerful and stable, and magic being either very uncertain or weak. You can kind of compare the "feel" of the ages, also, to the seasons-Early Era is high summer, everything is bright and strong and magical, Middle Era is autumn, there's lots to eat, lots of babies, and everyone lends a helping hand, Late Era is Winter, everything's dying and everyone's sick, or atleast irritable, and the sun doesn't stay out too long, most days. Those with the brains and the money to do so are sitting around at home, inventing things.