17 is highly unusual, even in Eretz Yisroel and even among chassidim. Maybe 20 for chassidim. Are they defying Chazal?
No litvish Rosh Yeshiva is telling his talmidim to go out at 17 maybe a few are going along with 21, but many aren’t. So you have a choice; either nobody is going against Chazal, or they’re all going against Chazal.
According to Rabbeinu Yonah none are going against Chazal, but there maybe a difference of opinion as to when boys are typically ready to get married.
According to Rabbeinu Joseph and Rabbeinu Libbi they’re all going against Chazal.