Any person under the age of twenty-one who apart from this section would be committed to prison under section 10 of the M1Extradition Act 1870 (committal of a person alleged to have committed an extradition crime) or section 5(1)(a) of the M2Backing of Warrants (Republic of Ireland) Act 1965 (remand in custody of a person for whose arrest a warrant has or is alleged to have been issued in the Republic of Ireland) shall be committed to an institution to which he could be committed if he were charged with an offence before the court which commits him, and any reference in those provisions to prison shall be construed accordingly.