That sounds pelagian, or at least arminian/wesleyan. Here you contradict yourself

2. The age is not specified chronologically. Some people who are severely mentally handicapped and infants dying in infancy never reach that age.
Emphasis mine.

If your first statement is correct, and it is, your second statement is absurd. You claim there is no age, then claim an AGE that is never met. Maybe we should refer to it as a 'personal point of accountability'? Again, something I do not see in scripture. I also notice that on such an important thing as salvation of children, God is silent of an age, or a point. The Bible also says nothing of those with mental handicaps.

If Adam did not have true free will,

Could you provide some scripture that says Adams will wasn't bound to his nature?

One more question;

If infants are not held accountable, why do they suffer the consequences of sin?

God bless,