How many prayers must be made up due to masturbation without knowledge of the necessity of ghusl, from the age of six until the age of eighteen?

A ghusl (ritual bath) is not necessitated by the mere act, but rather by the emission of semen, the description of which is detailed in Fatwa no. 102977. If its emission is confirmed, then ghusl becomes obligatory; otherwise, it does not. If you prayed ignorantly of thel's obligation, there is a difference of opinion regarding the necessity of making up the prayers. You may adopt the view of Ibn Taymiyyah, who holds that it is not obligatory. However, if you wish to be cautious, you should make up the prayers whose number you are unsure of by estimation until you are reasonably certain of having fulfilled your obligation, as stated in Fatwa no. 72777.