Almost every tile roof leak we attend in Perth starts in the same place, and it is not the tiles.
Ridge capping sits on a bed of mortar and is sealed with a flexible pointing compound over the top. The bedding holds the cap in position. The pointing keeps water out of the bedding. Perth sun works on the pointing until it goes brittle, then cracks, and once water reaches the bedding, the mortar breaks down and the capping starts to move.
You can usually see it from the ground. Look along the ridge line for hairline cracks in the mortar, a cap that sits slightly out of line with its neighbours, or mortar crumbs in the gutter after rain. Any of those means the clock has started.
The fix is re-bedding and re-pointing, which means lifting the capping, laying fresh bedding and applying a flexible pointing compound rated for the movement. Done properly it lasts another fifteen to twenty years.
What it is not is a reason to replace the whole roof. If a contractor looks at cracked pointing and quotes you a full re-roof without discussing re-bedding, get a second opinion.