I just googled and the UN defines it as 15-24.
https://en.m.wikipedia.org/wiki/Youth
No fair. You googled.
My 4 kids are 17 -25. I would agree with up to 24.
My kids have been raised rather conservatively compared to other kids they know (from dysfunctional homes, raised by grandparents, families scraping by always, etc.)and in a suburban area, without much in the way of city smarts. So they are naive on some things in the world and learning as they are exposed to it.
Oldest son married at age 23, is now 25, and lives in a major city on their own and travels weekly to other major cities. He studied abroad for 5 weeks one summer. So I definitely say he's not a youth anymore.
Our 22 year old is less outgoing and lives a quieter homebodylife. He's in year 5 of a 7 year college program. Still has some things to learn and navigate about the world, but I am confident he'll be okay by the time he graduates and gets a real job.
I do think some of the cultural push for kids to go to college has slowed some kids down in achieving adult status sooner.
My parents generation and the 2 generations before, a lot of kids were drafted out of high school or off the family farm and sent to war. They had to grow up in a hurry.