What counts is the age a child is at check-in -- and they know that because you are required to give the birth date of every person in your party when you book a Disney resort.
The ages are 3 thru 9 for a "child" and 10+ for "adult." This is the age for tickets and for dining.
What I find the most useful is to carefully plan out the use of my credits. I have an Excel spreadsheet I use to determine each day's meal. It calculates for me the number and type of credits used and I can play and rejigger and replan until I get it all worked out that I've used up my credits. (Although, I *have* left some snack credits until the last day, when I use them up by buying snacks for the plane.)
If it helps, here's a link to my Excel spreadsheet:
http://www.remleml.com/mouse/downloa...-DDP_v_DDE.xls
Eileen
PS - if a child is 9 when you check-in, but turns 10 during the trip, Disney considers that child a "child" for the duration of the trip.