At what age does one cease to be a “youth”?

[Santander]

It phases out between 25 and 35. 25 is the age where people start expecting you to own your sh**t, and at around 35, you're considered a proper "grown up". I chose 30 as the midpoint.

[Santander]

I always thought "youth" referred to under 18?

No, that's "child". (in most jurisdictions) Depending on context, the cutoff for youth can be different - the age your brain is fully developed, the age at which you're no longer considered "just starting out" in your life/career, the age at which you have to pay full price for a golf club membership, the age at which you have to start getting your own healthcare insurance, various government cutoffs... it all depends. 25 is probably the most common cutoff.