Once a Catholic always a Catholic? According to the Roman Catholic church, what is the status of someone who was baptised as a baby and left the church as a teen? Does it matter if he had Confirmation? Does it matter if he claims no religion or if he converted to another one?

I've heard that "once a Christian always a Christian", but I've also heard that people who die not in a state of Christian grace are in trouble. What does the church teach about a person who leaves?
I'll start with the "state of Christian grace" bit. The common teaching (and probably not limited to Catholics) is that people who die not in a state of grace do not go to heaven. BUT, the church also teaches that we don't know who is in a state of Christian grace, or not.

One of the teachings of the Roman Catholic church is that once a person has been baptized, he or she remains baptized for life.  The church also teaches that just not going to church is not sufficient to leave the Catholic church; in order to leave, one must formally renounce one's membership in the church, but even this does not undo baptism.

The church would say that one who has renounced membership will probably not be in a state of grace at the time of death, but that there is no way for a human to know for sure whether the dying person is, or is not in a state of grace.