Never.
My parents took my oldest brother on a trip when he was a toddler. My father took my other 2 brothers on trips to visit his mother in Florida; I was never taken on one of those (though he DID take me and one brother on an overnight train trip when I was 9). Other than those, we basically only went on vacations every 2 years, and always to the same place (oceanfront city about 4-5 hours away). First time I ever flew was when I was 17, and visiting colleges - Dad took me to several, then had a work event he had to attend so he sent me home via airplane.
I remember being worried I’d need to get my luggage at the airport where I changed planes, and was very gratified to learn that I could check it all the way through to our home airport.
My kids have been much better traveled than I was at any point. We took Dweezil on his first plane trip when he was about 18 months old - to visit family. These days, that same trip, we’d just drive - with airport delays etc. driving would be faster. Another trip to Boston when they were 4 and 1. Then we took them to California when they were 8 and 5 respectively, and to Arizona when they were 11 and 8, and to the southwest again when they were 16 and 13.
So: 3 or 4 trips apiece, very sporadic.
Ooooh yeah.
Well, there was the time I had to fly to Florida on very short notice (like, 12 hours), to drive my in-laws north. Then again, this was when a hurricane was approaching, and for some reason, there were NOT that many people flying TOWARD it. We actually had an empty middle seat.
And when we flew to Arizona in 2005, the plane was not crowded - to be fair, that was December 25th. The return flight, on January 1, was jammed.
In the early 1990s, I was basically commuting to California every week. I changed planes either in Dallas or Chicago. A surprising number of those flights were fairly empty, and I was able to stretch out across several seats.