Section 24. If a convict is sentenced to the state prison, except as an habitual criminal, the court shall not fix the term of imprisonment, but shall fix a maximum and a minimum term for which he may be imprisoned. The maximum term shall not be longer than the longest term fixed by law for the punishment of the crime of which he has be convicted, and the minimum term shall be a term set by the court, except that, where an alternative sentence to a house of correction is permitted for the offense, a minimum state prison term may not be less than one year. In the case of a sentence to life imprisonment, except in the case of a sentence for murder in the first degree, and in the case of multiple life sentences arising out of separate and distinct incidents that occurred at different times, where the second offense occurred subsequent to the first conviction, the court shall fix a minimum term which shall be not less than 15 years nor more than 25 years.

In the case of a sentence of life imprisonment for murder in the first degree committed by a person on or after the person's fourteenth birthday and before the person's eighteenth birthday, the court shall fix a minimum term of not less than 20 years nor more than 30 years; provided, however, that in the case of a sentence of life imprisonment for murder in the first degree with extreme atrocity or cruelty committed by a person on or after the person's fourteenth birthday and before the person's eighteenth birthday, the court shall fix a minimum term of 30 years; and provided further, that in the case of a sentence of life imprisonment for murder in the first degree with deliberately premeditated malice aforethought committed by a person on or after the person's fourteenth birthday and before the person's eighteenth birthday, the court shall fix a minimum term of not less than 25 years nor more than 30 years.