Load bearing capability of joists.

Determining the load-bearing capacity of joists involves various factors, including the type of wood, spacing between joists, and the specific conditions of your structure. It's crucial to consult with a structural engineer or a building professional who can assess your specific situation. They may need to consider additional factors such as existing loads, the overall structure, and the condition of the joists. Attempting to load a stud wall with tiles without proper consideration of the structural capacity could lead to safety risks and potential damage to the building. Always seek professional advice to ensure your modifications comply with safety standards and local building codes.

Due to the condition stated, i.e., the age of the structure, I highly recommend consulting a professional. Alexander Vaudejes, Construction Engineer

The floors are already calculated to support any partition wall, with or without tiles. I suggest to do someone noggins and build your wall with no problem. If you want, you can double the joist by screwing a new one on the side.