I don't think the system will automatically recognise the age of mp members. You will have to click on "children" beside the number of adult travelers. That will lead you to a page with 3 categories of children and 2 categories of infants travellers. With that, you should get it priced right next time.
If MP could remember ages it would stop asking me if I have any children traveling with me on my one-person itineraries at check-in.
