Dear Jerry:

The short answer to your question is "no". The determination of mortalness of a sin is determined at the time the sin is committed. A venal sin does not change into a mortal sin.

In order for a sin to be considered mortal, three things must be true (all three of these factors must be present to consider a sin mortal):

1) The sin must be grave. This is an objective determination. The sin either is or is not grave. The Catechism states:

1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother." The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger. 

2) The person must have full knowledge that the sin is grave.

3) The person must give deliberate consent to the sin.

The Catechism states:

1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin.

1860 ...Sin committed through malice, by deliberate choice of evil, is the gravest.

It is unlikely that even a ten year old boy does not have knowledge of the serious sinfulness of stealing a car. The element the boy is most likely to fail is complete consent. Immaturity can, but not necessarily, diminish the culpability of a sin. 

The Church does teach the concept of "Diminished Responsibility". Here are the paragraphs on this in the Catechism:

1735 Imputability and responsibility for an action can be diminished or even nullified by ignorance, inadvertence, duress, fear, habit, inordinate attachments, and other psychological or social factors.

1860 Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders...

2352 (concerning masturbation, but nevertheless a principle is given that can apply to other sins)...To form an equitable judgment about the subjects' moral responsibility and to guide pastoral action, one must take into account the affective immaturity, force of acquired habit, conditions of anxiety, or other psychological or social factors that can lessen, if not even reduce to a minimum, moral culpability.

The degree of diminished responsibility only God can really know. We can suspect it in others or in ourselves, but only God can really know. One should NEVER make the judgment that he is in a state of diminished responsibility by himself. This needs discussion with a wise confessor. But, even then, confession of the sin anyway is the wiser course.

So a good rule of thumb is to Confess all grave sins without regard to whether or not one may have diminished responsibility.

As to the boy in your scenario, he ought to confess the sin not because the sin changed from venial to mortal (that is not possible), but because the sin was grave in the first place and should be confessed. The boy "may" have been in a state of diminished responsibility, but he cannot know that for sure. It is better to confess the sin. 

If the man comes to realize that he never confessed this sin of his youth, then he can confess it now, though all forgotten sins of the past are forgiven when the priest absolves you, yet it one comes to mind later, one should mention it in the next confession even though it technically has already been forgiven.

God Bless,
Bro. Ignatius Mary