In the Name of Allah, the Beneficent, the Merciful, and blessings and peace upon our master Muhammad, his family, companions and those who follow their noble way.
In the Maliki school it is not permissible to delay the obligatory prayer beyond its preferred (ikhtiyārī) time to the necessary (darūrī) time without a valid excuse.
A valid excuse, as Imam Dardir mentions by way of example in Sharh Sagir, would be such at the following:
1) Disbelief for one entering Islam or returning to Islam.
2) Pubescence for a child who experiences the physical changes from childhood to maturity (such as ejaculation or menstruation).
3) Unconsciousness or Insanity.
4) One unable to find water or clean earth for purification.
5) Menstruation or post-natal bleeding.
6) Sleeping or forgetfulness.
Other reasons could include illness, incontinence, or travel.