Is that really true? My commercial airline pilot uncle didn't fly a glider until after he retired, but that's just anecdata.

I used to follow Patrick Smith, aka "Ask The Pilot". He didn't start with a glider.

Now I follow Scott Manley. He just got his private pilot license, and did not start with a glider. Nor did my missionary pilot uncle. Nor did a co-worker of mine who was learning to fly.

Hereabouts I see gliders sometimes, but I far more often see powered aircraft. I get the impression there are far more powered flight schools than glider schools; https://www.flightschoollist.com/california-glider-schools/ lists 13 glider schools in California while https://www.flightschoollist.com/california-airplane-flight-... list 91 for powered flight.