The Leviim wash the hands of the Kohanim when they go up to bless the people. If a Levi is not available a first born (to the mother) can wash (according to the Askenazi custom) instead, Orach Chaim 128 (6) [MB 22].
Questions:
1) Does the Levi have to be over bar mitzvah?
2) If over bar mitzvah is not required and there is a Levi over the age of chinuch and a first-born over bar mitzvah, who gets the job?
Related Why do the Leviim wash the hands of the Kohanim when they go up to bless the people? and Washing Kohen's Hands