Is it obligatory to re-perform the prayers for someone who used to practice masturbation during adolescence and was unaware of the necessity of performing ghusl after ejaculation, contenting himself with wudu' only? And what are the implications of this for fasting Ramadan?

As long as you are not certain that semen was discharged during your practice of this bad habit, you are not required to make up any prayers or fasts, because the default assumption is that it was not discharged, and doubt about the discharge of semen does not necessitate a ruling.