The problems that come from withholding original sin

Original sin is a doctrine that's somewhat difficult to come to grips with, but if you consider it carefully you'll see that refusing to believe in it creates a number of really difficult problems.
 
1. If people begin life perfect then at what point do they cross over into the age where sin begins to count against them, typically called the ‘age of accountability?’ If you reply "it is different for everyone" then what's the average age? Are there sins so heinous that God would count them when committed by someone still under the age? If God is just then why does He allow sinless children to die if death is the penalty for sin? Questions here could be multiplied.  

2. If everyone starts without fallen natures then why is sin in every single heart? Why is every last society, man, woman, and child broken?

3. Why, if the Bible is sufficient to tell us everything we need to know, does God refuse to disclose the age of accountability to us? Why would He withhold this critical information? We have an oblique reference in Isaiah and Jonah but that's it?
 
4. If babies go to heaven because they don't understand (or don’t knowingly commit sin) then what's to stop adults from getting a free ride on the same train? If God is willing and able to turn a blind eye to wicked creatures in one case why might He not do so elsewhere?  Since ignorance does not count against us, is all we need to forget about our sins to be saved? This is tantamount to insisting that Jesus was superfluous, and His death for our sins was merely a nice idea, not an essential event.

5. A man without original sin is a man capable of achieving salvation. But scripture does not allow us of this, instead it speaks in the strongest terms of our moral inability and fallen hearts: But we are all as an unclean thing, and all our righteousness are as filthy rags; and we all do fade as a leaf and our iniquities, like the wind, have taken us away (Is 64:6, Eph 2:8,9) For by grace are you saved through faith; and that not of yourselves: it is the gift of God: Not of works, lest any man should boast.”

6. A rejection of sin directly contradicts scripture: “The wicked are estranged from the womb: they go astray as soon as they be born, speaking lies" (Ps. 58:3). “Behold I was shapen in iniquity, and in sin did my mother conceive me" (Ps. 51:5).

7. Someone with a clean slate has no need of grace, and therefore no need of Christ's atonement for sins. Babies are therefore saved apart by their own nature, and not Him who took on our nature. 

I understand that the thought of being personally evil is repulsive to the flesh, but we must suppress this desire to justify ourselves and instead insist that people are tainted to the core, and that God is a merciful God. As the Scripture says, "Let God be true and every man a liar."