subscripted aging chain with 2 stocks

[sarahboyar@gmail.com]

Hello,
I am trying to find an elegant way to work with an aging chain but spread it over 2 stocks, children and adults.

So the stock for children[all but youngest] =
+births children population[all but youngest]
+children aging[previous cohort]
-aging[all but youngest]
+aging[previous cohort]
-deaths children population[all but youngest]
+net migration children[all but youngest]
*same equations basically for age = o = births, 2 equations for the stock.

But when I add an outflow for only the 18 years olds, moving into the stock with adults, i get drip into the 18 year olds in the children's stock over the year. I can't make a direct infinite delay from births to turning 18 because things happen along the way. Is there an elegant way to do this? I don't think a conveyor works either....
some adults always remain as children!! lol

[tomfid]

I assume the outflow is just aging[age18] ? I think we need more of the equations, or preferably a small model example.

Is the TIME STEP essentially discrete (i.e. same as the age increment in the subscript)?