The following is a narration of something that had happened in the past.
My friend says to me, "What did you tell him yesterday?" In reply, I said, "I told him that I could do it when I was young and that I couldn't do it anymore because I'm old now."
Since I'm talking about something that I said in the past, I changed "can" to "could". Still, I'm unsure if I should change "couldn't" to "can". Should I keep "couldn't", and should it be "I was old" because it's something I said in the past?
I read the other threads they're different from my question.