Kant maintain the ethics must be based purely on reason; why ? Why is it a mistake to argue from contradictory premises ?

Kant makes a strong argument because someone who has a
very low IQ may know what right from wrong is but he/she would be unable to make a
decision based on ethics.  Instead, he/she would just be following a directive.   In
this same manner one must lo at a teenager making a decision about marriage.  The teen
only has certain details of information and up until a certain level the teen has not
gained all of his cognitive abilities.  His reasoning is obstructed by his
youth.

To be able to make an ethical decision one must be
able to be cognitively aware of the pros and cons and consequences for oneself or others
based on the decision. Therefore, Kant theory about reason is basically
correct.