Roofs don't last forever. It all depends on the materials used, the location of the roof (which way the building faces, if there are trees near or overhanging, etc), and the way the roof has been maintained. The nails fixing the tiles on can also rust away or the battens can start to sag.
If you are finding that you are constantly suffering broken or dis-lodged tiles, or are finding bits of roof cement in the gutters or on the ground around the house, it's probably time to consider having the roof stripped and re-tiled.
A strip and re-tile encompasses us removing all the roof tiles, roof battens and roofing felt, and then applying a modern roofing membrane, new roof battens, and then re-fitting the roof tiles (replacing any that are damaged with matching re-claimed tiles).