What should I do regarding the transgressions I committed in my youth, believing they were expiation, and I would perform ghusl and recite the Shahada afterward, and I do not know whether I was an adult at that time or not?

Since you have repented from theibilities you considered disbelief, your repentance is valid, whether those acts were indeed disbelief or not. There is nothing further required of you now, regardless of whether you were an adult at the time or not. The default assumption is that you were not an adult if you are unsure. Repentance wipes out what came before it, and the one who repents from a sin is like one who has no sin. You did not mention the sins you committed for us to clarify whether they necessitate apostasy or not.