Advantageous to be young with a lisp … (6)
I believe the answer is:
useful
'advantageous' is the definition.
(something advantageous is useful)
I don't understand how the remainder of the clue works.
(Other definitions for useful that I've seen before include "Capable of serving some purpose" , "Proficient; handy" , "Handy, serviceable" , "Make yourself . . . . . . - help out" , "Handy; very able" .)