All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
First of all, we praise Allah Who guided you and we ask Him to make you firm on the truth and increase your guidance.
You should praise Allah that He guided you while you are still very young and your register is not yet filled with abandoning religious obligations.
As regards making up for what you had missed of Ramadan after puberty, then according to the view of the majority of the scholars, you are obliged to make up for whatever days you had missed since you reached puberty.
If you do not know exactly when you have reached puberty, then you have to make up for whatever you are sure that you had abandoned after reaching puberty.
If you doubt whether you have reached puberty at the age of eleven or twelve, for example, then consider that you have reached puberty at the age of twelve and make up for the days that you had missed from Ramadan of that year.
Since you write to us in Arabic sometimes, then we refer you on this matter to Fatwa 295363 in our Arabic website about the ruling of making up missed obligations for whoever does not know exactly when he has reached puberty.
For more benefit, please refer to Fatwa 356176 in our English Site.
Allah knows best.