The strings are due to change, especially if they haven't been replaced in over 20 years. Over time, steel strings lose their tone and can become brittle, making them more prone to breaking. I’d recommend switching to a fresh set of steel strings. For an acoustic guitar, a popular choice is the "D'Addario EJ16" phosphor bronze light gauge (12-53) for a balanced sound, or you could try "Elixir Nanoweb" for longer-lasting strings.