OK, one of my coworkers figured it out. The magic word was the aggregate command. This works

with member [Ages].[Age].[Under 18] as aggregate([Ages].[Age].&[0] : [Ages].[Age].&[17])

member...

That did it! Yes, I had to build out the entire range as in

with member [Ages].[Age].[Under 18] as ([Ages].[Age].&[0] + [Ages].[Age].&[1] + [Ages].[Age].&[2] + [Ages].[Age].&[3] + [Ages].[Age].&[4] + [Ages].[Age].&[5] +...