Your conscience has a way of telling you when something is wrong. However, if it were possible that at some point you were absolutely unaware of the sinful nature of an act, then there’s at least no mortal sin. For there to be a grave sin, there needs to be three things:
- A grave matter: the act must be something serious not just any petty mistake or action.
- Full knowledge: The person must be fully aware of the seriousness of the said action and the sinfulness.
Delibrateconsent: The person must choose freely and without coersionor influence. Any impediment to free choice diminishes or even possibly removes the guilt of the sin. For instace, a mentally challenged person cannot be guilty of anything they do when they’re unable to grasp anything.
If any or all of these aren’t present in an act, then the sin isn’t mortally sinful. As to whether or not one is guilty at all, even of venial sin, i cannot say. But the general consensus is: God pardons inculpable ignorance. Which means, if a young person commits a crime they don’t know and also shouldn’t know at their age, then there’s no sin at all. But if the ignorance is culpable, then the person is guilty of at least venial sin. Since they commit a sin they don’t know is sinful but should know.
The last part is only my own opinion.
God bless you.