ANSWER:

If you have reached the age of puberty, these missed fasts are mandatory upon you and you MUST make them up no matter what anyone says as there’s no pleasing or obedience to anyone at the cost of disobedience to Allah.

Be diplomatic, respectful and humble with your parents in all situations while not compromising on any of the rulings of Islam.

I am a 12 year old boy and i think i reached the age of puberty because i can see some changes . So this ramadan Alhamdulilah i fasted almost the whole month but missed 3 days because of illness . So i wanted to make them up later after ramadan was over , but my father strictly prohibits to do so , because most of cousins , including my father say i have become lean or thin