What is the ruling on someone who performed anal sex during the day in Ramadan at the age of 17, and is he required to make up the days given that he does not remember the number of times, and are his fasts in subsequent Ramadans valid?

You must repent to Allah for what happened. It is not permissible to insert anything into the anus. You must also repent for violating the sanctity of the month of Ramadan. The fast is not broken if the finger was not wet. If it was wet, the more correct opinion is that the fast is not broken. However, if semen was ejaculated, the fast is broken. If there is doubt about its ejaculation, the default is that it was not ejaculated, and no qada' (making up the fast) is required of you. What you did falls under prohibited masturbation, and sincere repentance is obligatory. There is no breaking of the fast if ejaculation did not occur.