This would apply to SOME things, such as the precepts of the Church: If you were never Catholic, obviously you would not be bound to attend Mass on Sundays and Holy Days of Obligation or to support the Church, etc. These are laws that specifically apply to Catholics.
If, however, you have committed murder, adultery, stuff like that, these are sins that must be brought to Confession, regardless of whether or not you were Catholic when you did it. You must be in a state of grace (having brought all this junk to Confession, without deliberately holding anything back) in order to receive Confirmation or Holy Communion in the Catholic Church. You do NOT have to go to Confession before receiving Christian baptism, because that wipes the slate clean; but if you became Anglican at 15, may I assume that you were also baptized?
The three things necessary for mortal sin:
It must be grave matter (serious stuff - like ‘I had an abortion’, not ‘I stole candy’).
It must be done freely (you weren’t forced to do it, you had a choice - no one can be forced into commiting a mortal sin, even if it involves grave matter).
You must know that its wrong (its one of those “bad-thing-to-do” things. You can’t commit mortal sin in your sleep, or if you don’t have the mental capacity to properly judge right from wrong; you have to know what you are doing).
If any of these three is missing, you have not committed mortal sin. However, if there is the reasonable possibility that the matter was serious, you knew it was a “bad-to-do”, and you consented freely, definitely bring it to Confession.