RegattaCentral

Here is a link to U.S. Rowing's current Rules of Rowing: Rules of Rowing - USRowing

As for our newly added U15 and U17 age categories:

(b) The following classifications are used to determine a Competitor’s eligibility based on age: 
...
(4) U17: A Competitor may compete in the U17 category until December 31 of the year of their 16th birthday. 
(5) U15: A Competitor may compete in the U15 category until December 31 of the year of their 14th birthday.

(c) Youth: A Youth is a Competitor who in the current calendar year does not attain the age of 19, or who in the current calendar year does not attain the age of 20 and has been continuously enrolled in a secondary school as a full-time student seeking a diploma. A Competitor thus ceases to be a Youth after December 31 of the year of their 18th birthday (U19), or upon completion of the 12th grade of secondary school, having been a full-time student within the year of their 19th birthday (U20). Athletes older than U20 are never eligible to compete as Youth.