It’s undeniable that briefs are associated almost exclusively with young boys and old men. It’s quite rare to find teenage boys, young adults and even men in their 30s, 40s, 50s etc. who wear briefs, I definitely think briefs are slowly dying out.
As a swimmer, I’ve spent many hours over the years in changing rooms and even a lot of older men now wear boxers. I haven’t seen anyone over the age of like 9 or 10 wearing briefs in changing rooms for years and years.