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• "Adults in age, not in reason": Billot argued that a great deal of the world's population, despite being "adult" on a biological and civil level (capable of managing daily functions), has not reached the level of intellectual and moral development required to perform acts of conscience that engage their responsibility before God.
• The parallel with children: According to Billot, these individuals, though adults, are "morally children." They do not possess the fullness of the use of reason and moral free will necessary to commit mortal sins (in the theological sense of a conscious rejection of God) or to perform salvific acts of faith.
• The consequence (Limbo): Since they lack the capacity to sin gravely but have not received baptism, Billot concluded that they cannot be damned (as they have no personal guilt), but neither can they access the beatific vision (Heaven). Therefore, upon death, they would enter a state similar to the limbo of children, where they would enjoy a purely natural happiness.

• Egocentric reactivity: The person evaluates events solely according to their direct impact on their personal comfort, lacking the capacity for cognitive empathy toward others or for a notion of a true right or wrong outside of negative consequences to them.
• Absence of long-term thinking: A chronic inability to sacrifice immediate gratification for the sake of distant values or goals, even in a person who "manages their finances" (e.g., compulsive spending despite apparent savings). The person lives almost entirely in the “here and now”
• Avoidance of responsibility: As soon as a mistake is made, the person systematically seeks an external scapegoat (circuмstances, other people), remaining incapable of introspection or self-questioning.
• Dependence on external validation: Their "morality" is dictated by what is socially acceptable or "trendy" at the moment, rather than by anchored principles.
• Binary thinking (black or white): Inability to understand nuances, compromise, or the complexity of human motivations. Everything that is not "with me" is perceived as an attack.
• Low resistance to peer pressure: Although an adult, the person changes their opinions or behaviors instantly to align with their immediate social circle to avoid exclusion.
• Absence of authentic introspection: The inability to stop and look at themselves and character, to see what they’re doing right or wrong and what they should change. They have the ability to ask for forgiveness to calm a situation and restore the status quo, but without any subsequent change in behavior, due to a failure to understand the harm caused.
• Disengagement from consequences: A striking indifference to the collateral damage caused by their decisions, as long as they are not personally inconvenienced.
• Total external locus of control: The conviction that the world is a series of events happening to them, rather than a reality upon which they can act through conscious ethical choices. They feel as if their own choices are things which “happen to them”.
• Manipulation as a survival tool: The use of emotion (tears, anger, victimization) as a lever to get what they want, rather than through adult negotiation based on reason and mutual respect. They may implicitly see words not so much as a way to convey information and concepts but as a series of sounds to make in order to get what they want.
• Inability for the spiritual: The person cannot grasp the importance of existential questions like God, the afterlife or the meaning of life, they sincerely see it as “overthinking”.

"has not reached the level of intellectual and moral development required to perform acts of conscience that engage their responsibility before God."

While certainly there are some individuals who fall into this category and have mental defects, I think the majority of people cannot use this as an excuse. It sounds a lot to me like "he's on the spectrum". It becomes an excuse for the "child" (adult child or teenage - age child) to get special treatment and shirk responsibility.

You know much of what is right or wrong at a young age from the natural law. Morality is objective and not subjective. When it comes to sin, the matter of the sin is objective - the culpability of the individual can vary based on their ignorance or the circuмstances, but remember the catechism. There is vincible and invincible ignorance. If we don't do our duty because we are content to be ignorant, or we don't want to know whats right or wrong, it actually increases the culpability. (St Tomas Aquin I believe) If we are retarding our our mind, and God asks an account of us at our judgement, it's our own fault. Parents will suffer if they have not done their part in helping form a correct conscience in their children, but at some point, it is on the child to be an adult.

We have a duty to develop our conscience and conform it to the higher law (God's law). Our conscience is like a wristwatch. If we are not checking it against the master clock which is God's, it will drift more and more out of synchronization. We have to calibrate ours with God's. We have to seek out the truths - whether by reading or consulting our parents/priests, etc. Prayer is also essential. If you don't feed the fire, it goes cold. A watch that doesn't tell right time is useless. You'll follow your own "whims" or "conscience " wherever it goes, but it will lead you with no reason behind it unless you correct its course. 

Signs of "Moral Immaturity" in a Modern Adult
Transposing the theological concept of being "morally a child" to a modern adult person in the United States requires shifting from a metaphysical register to a psychological and behavioral one. Although such a person may be functional (in work and finances), moral immaturity can manifest as an inability to integrate abstract ethical principles into one's decisions.
Here are eleven examples of signs which may indicate that a person might have this type of moral "blockage":

• Egocentric reactivity: The person evaluates events solely according to their direct impact on their personal comfort, lacking the capacity for cognitive empathy toward others or for a notion of a true right or wrong outside of negative consequences to them.
• Absence of long-term thinking: A chronic inability to sacrifice immediate gratification for the sake of distant values or goals, even in a person who "manages their finances" (e.g., compulsive spending despite apparent savings). The person lives almost entirely in the “here and now”
• Avoidance of responsibility: As soon as a mistake is made, the person systematically seeks an external scapegoat (circuмstances, other people), remaining incapable of introspection or self-questioning.
• Dependence on external validation: Their "morality" is dictated by what is socially acceptable or "trendy" at the moment, rather than by anchored principles.
• Binary thinking (black or white): Inability to understand nuances, compromise, or the complexity of human motivations. Everything that is not "with me" is perceived as an attack.
• Low resistance to peer pressure: Although an adult, the person changes their opinions or behaviors instantly to align with their immediate social circle to avoid exclusion.
• Absence of authentic introspection: The inability to stop and look at themselves and character, to see what they’re doing right or wrong and what they should change. They have the ability to ask for forgiveness to calm a situation and restore the status quo, but without any subsequent change in behavior, due to a failure to understand the harm caused.
• Disengagement from consequences: A striking indifference to the collateral damage caused by their decisions, as long as they are not personally inconvenienced.
• Total external locus of control: The conviction that the world is a series of events happening to them, rather than a reality upon which they can act through conscious ethical choices. They feel as if their own choices are things which “happen to them”.
• Manipulation as a survival tool: The use of emotion (tears, anger, victimization) as a lever to get what they want, rather than through adult negotiation based on reason and mutual respect. They may implicitly see words not so much as a way to convey information and concepts but as a series of sounds to make in order to get what they want.
• Inability for the spiritual: The person cannot grasp the importance of existential questions like God, the afterlife or the meaning of life, they sincerely see it as “overthinking”.