Must I make up the Ramadan days I missed from an early age (after my father made me understand that I was permitted to break my fast due to work) until the age of thirty, and how are these days to be calculated, knowing that I am now seventy years old?

You must make up for the fasts you missed from previous Ramadans, along with repenting to Allah for this negligence. Fasting these days is a debt upon you, and Allah's debt is more deserving of being fulfilled. Your ignorance and reliance on your father in this clear matter are not an excuse that absolves you from making up the fasts. You must fast and estimate the days to the best of your judgment. If you are unable to make them up, then feed a poor person for each day you did not fast.