The form to fill in which I think you can down load from the DGT website (Dirección General de Tráfico)
The old license
to do the "psicofisico" tests - eyesight, reaction time etc
2 photos, one of which has to be put on the "psicofisico" test report
The original residents permit/ certificate (Don't know if you need a photocopy or not)
The good news is that as you're over 70 you don't have to pay tax.
I'm NOT absolutely sure this is right, but you WILL need all of these things, I'm just not sure if you need anything else. If you don't have a DGT office near you try going to a driving school and asking there. If you don't speak Spanish show them your old license and ask ¿Cómo renovar?
Hope you're successful!