Is expiation and making up the days (Qada') obligatory for someone who masturbated during the daytime in Ramadan when he was 16 years old, believing that fasting was not obligatory for him? And does sin prevent sustenance?

One who reaches sixteen lunar years of age is held accountable for their actions. Masturbation is forbidden and does not invalidate the fast unless semen is discharged, in which case the fast must be made up (qada') without expiation (kaffara). Expiation is only required for invalidating the fast through sexual intercourse. Regret is repentance, and he who repents from a sin is like one who has committed no sin. Strive to earn a lawful livelihood, for everything is by divine decree.