because of their constitutional functions. Lord Grantham will need to make provision for the possibility that he will die before Baby Boy Crawley is 21. The normal way would be to arrange for the money and estate to be held and managed in trust until the new Earl (he would inherit the title at once) came of age. There would be nothing to prevent Mary being her son's Trustee, but there would usually be more than one and she would have no automatic rights. Of course it is possible that Matthew's Will, if he had one (and given his profession he certainly should have thought of it when he married, which in itself would negate any previous Will he had made) left his money to Mary, which would alter the balance of power somewhat.