We have had an 18U team for about 8 years. Get about 3 - 4 each year out of 14U and hold on to them until they age out. Pretty much the same methodology as a 4 year college program.
Around here a "true 16u" team is scarce, most jump from 14 straight into 18u. The true 16's usually have to play up in a combined 16/18 anyway, so why handicap yourself from the gate.