When does sin start?

What is sin? Sin is lawlessness or breaking the law:-

1Jn 3:4 Everyone who practices sin also practices lawlessness, for sin is lawlessness. (MKJV)

1Jn 3:4 Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law. (KJ)

The Law explains what sin is:-

Rom 3:20 Because by works of Law not one of all flesh will be justified before Him, for through [the] Law is full knowledge of sin. Psa. 143:2 [ISV]

Rom 7:7 What shall we say then? Is the law sin? Let it not be said! But I did not know sin except through the law. For also I did not know lust except the law said, You shall not lust.

Rom 7:8 But sin, taking occasion by the commandment, worked in me all kinds of lust. For apart from law sin was dead.

Jesus said the Law refers to the commandments:-

Mat 5:17 Do not think that I came to annul the Law or the Prophets; I did not come to annul, but to fulfill.

Mat 5:18 Truly I say to you, Until the heaven and the earth pass away, in no way shall one iota or one point pass away from the Law until all comes to pass.

Mat 5:19 Therefore, whoever relaxes one of these commandments, the least, and shall teach men so, he shall be called least in the kingdom of Heaven. But whoever does and teaches them, this one shall be called great in the kingdom of Heaven. [ISV]

These commandments are the Ten Commandments given in the Old Testament:-

Rom 13:8 Do not continue to owe no one, nothing, except to love one another. For the one loving the other has fulfilled the Law.

Rom 13:9 For, "Do not commit adultery," "do not murder," "do not steal," do not bear false witness, "do not lust," (from Ex. 20:13-15, 17)and if there is any other commandment, in this Word it is summed up, in the Words, "You shall love your neighbor as yourself." (from Lev. 19:18)

Rom 13:10 Love does not work evil to the neighbor. Then love is the fulfillment of Law.

The scribes and the Pharisees knew that the Law meant the Ten Commandments, and that committing adultery was breaking that Law:-

Joh 8:3 And the scribes and the Pharisees brought to Him a woman having been taken in adultery. And standing her in the middle,

Joh 8:4 they said to Him, Teacher, this woman was taken in the very act, committing adultery.

Joh 8:5 And in the Law, Moses commanded that such should be stoned. You, then, what do You say?

Wanting and yearning to sin, (even if no-one else knows), is the same as actually committing that sin :-

Mat 5:27 You have heard that it was said to the ancients: "Do not commit adultery." (See Ex. 20:14; Deut. 5:18)

Mat 5:28 But I say to you, Everyone looking at a woman to lust after her has already committed adultery with her in his heart. [ISV]

Giving in to wrong thoughts results in sin:-

Jas 1:13 Let no one being tempted say, I am tempted from God. For God is not tempted by evils, and He tempts no one.

Jas 1:14 But each one is tempted by his lusts, being drawn away and seduced by them.

Jas 1:15 Then when lust has conceived, itbrings forth sin. And sin, when it is fully formed, brings forth death.

In summary, all sin begins in the mind with uncontrolled thoughts that are not reigned in and brought under control.