Exposing one's sin is a very serious sin because of this Hadith:
Abu Hurayrah said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “All of my ummah will be forgiven except those who sin openly. It is a part of sinning openly when a man does something at night, then the following morning when Allaah has concealed his sin, he says, ‘O So and so, I did such and such last night,’ when all night his Lord has concealed him and the next morning he uncovers what Allaah had concealed.”
(Narrated by al-Bukhaari, 6069).
But what about if an adult exposed the sins he committed when he was very young, that is when he was not accountable for his sins (because of the Hadith that says the pen is lifted from three people: a child until he reaches puberty; an insane man until he comes to his senses; one who is asleep until he wakes up.).
For example, an adult says "I lied to my mother when I was 9 years old." Is this haram?