When should I change / replace my water pipe?
Note, since you don't provide your location, my answer is based on a US water supply and building materials.

To answer the question that you asked, you don't replace the pipes in your house until they fail in some way (leaking), present a health hazard (.e.g lead pipes), or you need to move them for a renovation. Pipes are installed inside of the walls in such a way that they are not easy to access, which is one of the reasons that plumbing problems can be a difficult and expensive repair.

For the questions you didn't ask:


If your fixtures are old, you may want to replace them with newer fixtures that will have new valves, clean aerators, along with a better look.
If your water is bad, filter it. This solves the problem of bad water from your pipes (rare), bad fixtures, or from your water supply.
If your kettle is rusty, replace it if the kettle itself is rusting. Otherwise, clean it and use filtered water.