I can remember two. One was in a car in the next lane at a traffic signal - a young woman (about 26-28 years old) wearing a high pull. The other was a young woman (about 20-22 years old) at Epcot at Disney World in Florida.

I seen a little boy probably about 15 wearing a high-pull headgear when I was on vacation. Only got a quick glimpse though

I saw a guy wearing one when on vacation in Vienna (Austria) a couple of years ago. Not sure if he actually qualifies as an adult, as he probably wasn't older than 17.

My girlfriend (who became my wife) had braces and headgear... at that time I thought I was the only one who had strange feelings about braces (this is pre-internet).

She never knew the effect they had on me, and even now knows nothing of my love of braces.

Does seeing yourself count? :-D

I've only ever seen one adult in headgear, many, many moons ago.  it was past me so fast I barely had time to register it.  Woman in her early 20's.

I have never seen a male or female person with headgear in the true life.

I've seen an (maybe 35 years old) male, with Family in the Netherlands.
He was to visit the Bataviawerf Lelystad. It seems he doesn't care about beeing in public with the Headgear.
He wore a Facebow with a red Neckpad and "Smartgear Safety Modules" with white stripes and a "Display" on one Side. I googled them. They're from Switzerland and beeing used to record wear time !!!
it was really fantastic to see this guy an his Headgear...