You should have started seeing boost appear on the enemy units in later rounds as early as HMA. As you move up in age, the enemy units not only move up as well but they get stronger and stronger bonuses. Players in Oceanic Future (last age currently) have to contend with an 85% boost on the enemy units from the very beginning, so you can get an idea of how much it's going to continue going up.
The expectation is that as you advance, you're also obtaining and levelling up GBs that boost your own army, and/or building military barracks that do the same.