If someone has a mental deficiency and passes away before experiencing a long life, will they still have a hereafter and participate in the Day of Judgment?
Allah is The Most Merciful on all His creatures. His Mercy on this person is much greater than your mercy on him. Allah will never punish a person who had mental deficiency which prevented him from knowing the truth and rules which are obligatory on usual persons. Allah’s Mercy will be much greater than our expectations.
Wassalam.