I'm 60, and never bother with the age group rankings. I have several KOMs and many top 10s ... and don't need the age comparison.

On the power meter question:
I've done a few rides that were recorded on both - my Garmin (which reads the power meter) and on my phone (which doesn't, and forces Strava to use the estimated power). that way I could compare the actual and the estimated power readings.

The details vary from moment to moment ... but the readings for the average power for the whole ride, and for longer, flatter segments, tended to be very close.