Jesus required one to believe [John 8:24] repent [Luke 13:3] confess [Matthew 10:32-33] and be baptized [Mark 16:16] in order to be saved.
So how is it possible for an adult to be sinless if he does not know who Christ is therefore does not believe? This adult may not commit any moral sins as lying stealing, murder adultery, but he has committed a sin in not believing repenting confessing Christ and being baptized as Christ commanded.
We can get back to the Scripture in a bit, but here is why your theology (let us call it the heresy of Seabasism for lack of a better term) is untenable.
1) In your view "original sin" consists of causing one man to pay a penalty (or be deprived of grace) because of the sin of another. You find this idea objectionable.
2) You believe that infants are born free of sin, and therefore all infants who die go to Heaven.
3) Because you deny original sin, you assert that man's nature is not corrupted. Thus, your recognize that a hypothetical man is capable of exercising free-will and choosing good over evil his entire life, without sinning.
4) We asked you about the logical conclusion that stems from (3) above, which is that our hypothetical man could live his entire life without sinning, and that therefore he would not need Jesus as a savior. That is, because he never sinned, there is nothing that he needs saving from.
5) To avoid the logical conclusion at (4) above, which should most certainly offend the ears of any Christian, you manufacture right out of thin air some sins for our hypothetical man to commit. That is, our hypothetical man has committed the sin of (a) unbelief, although there is no reason for him to believe that Jesus died for his sins since he has not committed any, (b) not repenting, although he has not committed any sins, (c) not getting baptized for the forgiveness of these uncommitted sins, and (d) not confessing Jesus as Lord and Savior, although there is no sin that he has committed.
6) In a previous thread concerning this same topic, I asked you about the fate of a person who has never even heard the gospel. You said that such a person was condemned, for the reasons stated above. That is, our hypothetical man would be lost for the sin of unbelief, and not getting baptized, etc., even in the case where he was born into a militant Muslim community (an ISIS camp, for example) and never once had an opportunity to hear the gospel.
7) So our hypothetical man is completely sin free and on the way to heaven up until the age of reason (the age of seven, for example), but at the very second that he reaches the age of reason, he instantly becomes a sinner on his way to hell for not believing something that has never even heard once, and for not being baptized although there is no person who would be willing to baptize him even if he desired it.
8) Even though you would say that our hypothetical man is being punished for this ridiculous sin of not believing something that he has never even heard, and for not being baptized when there is no-one to baptize him, in realty what your theology does is condemn our hypothetical man for the sins of his parents and the other members of the community in which he was born. That is, he is being punished for the sins of his parents and his community for not providing an environment in which he can hear and believe in the gospel, and in which he can be baptized.
9) So in effect, what Seabasism does is take the concept of original sin, and simply moves it to a different point in time. Seabasism does exactly what you object to, but at different times and with different people than in mainline Christian theology. Instead of Adam's sin being passed down to our hypothetical person at the moment he is conceived, your theology passes down the sins of his parents and his community to our hypothetical man, at the moment that he reaches the age of reason.