I graduated high school around 2008, so I'll shed some light on this.
Up until I was 12, I always wore white briefs. When I turned 12 or 13, I started wearing boxers. Mostly tartan boxers. 12/13yo was when I started middle school, and we had to change clothes for gym. This is also around the time boxer briefs started getting popular I think. I'd say roughly 70% of the guys wore boxers, the rest boxer briefs. No one wore briefs.
By high school, 65% of my underwear were boxer briefs, the rest boxers. I'd still say boxers were the majority rule of the school.
Now I'd say boxer briefs are probably what I see 75% of the guys in my age bracket and younger are wearing.
I think the change in underwear trends has a lot to do with, "I don't want to wear what my dad/grandpa wore." That's something every generation goes through with... well, everything. Whether it be clothes, activities, whatever. No one wants to do what, "old people" do. Example, most of the kids in my generations had mom's that drove suvs. We see suvs as a soccer mom car, we prefer crossovers. Our parent's drove SUVs because their mothers drove station wagons. See what I'm saying?
Like all things, underwear goes in cycle. I'm slowly starting to see briefs come back into fashion. As someone else said, it seems to be something that mostly Hipster kids are into.
If I had to do a break down of the kind of underwear different people wear I'd say (From what I've seen)...
- Boxer briefs: Guys in their teens up until their later 20s, sometimes might wear boxers.
- Boxers: Guys in their 30s up until the mid 50s.
- White Briefs: Guys in their mid 50s and up, maybe country boys.
- White boxers: Guys in their mid 50s and up
- Colored briefs: "Hipster" type guys in their 20ish
- Designer underwear: Depends on the brand. American Eagle is mostly younger surfer type white guys, Polo Ralph Lauren younger black guys, Calvin Klein would be gay guys, metro sexuals, then you have the brands that cater towards gay guys like Andrew Christian.
- Trunks: Mostly gay guys, over sexual straight guys that take a lot of shirtless selfies.
Again, this is just based off of what I've seen. Also, I live in the south.