that's funny. i replied to this thread hours ago, never got posted!
for array_multisort you would need to restructure your array. if you were going to this effort then it would be better (with your current structure) to create a new array of ages and unique ids (use the ids as the key), sort the new array the iterate over it to recompile the original array.
by far the best way is to do the array sorting implicitly in the sql query from which you retrieve the data (use and ORDER BY clause). then as you iterate over the recordset the data will be presorted.