You could equally well distinguish between BG and GB based on birth weight. Age has nothing to do with it. If the order of the children doesn't matter, the odds of conceiving a BG combination are twice as large as conceiving either a BB or a GG combination. The distinction is made so all combinations carry the same weight, which makes it easier to illustrate the solution.
As another alternative, you could distinguish between BG and GB based on a non-accidental property, like the alphabetic ordering of their names (assuming each starting letter is equally likely and no two siblings have the same name, both of which are probably false in practice).
As another alternative, you could distinguish between BG and GB based on a non-accidental property, like the alphabetic ordering of their names (assuming each starting letter is equally likely and no two siblings have the same name, both of which are probably false in practice).