I don't think there is a definite answer that applies to all age groups. There are some aspects a good coach needs at 10u that are not really necessary at 18u and vice versa. Conversely there are some aspects that are appropriate at all ages.
10u: needs a coach that has tremendous patience, an ability to teach the basics, ability to sell the parents on the time commitment, etc...
12u: patience is still important, basic and intermediate skills knowledge, motivation techniques (age appropriate), physical conditioning knowledge (age appropriate)
etc.
etc.
etc.