And until last year, most of the planes in the sky were also Boeing. And the Boeing planes in the sky, on average, are older.

For a fair comparison, this should be measured against flight hours per manufacturer

Takeoff/landing cycles, actually. Cruising at high altitude is much less rough on the airframe (including windows) than the strains that happen when changing altitude.

A quick search gave me 12K vs 10K planes. I couldn't find the actual hours or number of flights

Blatantly Overlooked Engineering Inadequacies, Now Grounded

Does Boeing also have more plans in service and/or more flights?

those don't mean much with out at least a "out of X planes from manufacturer Y"