In California, if someone has lived on your property for more than a year, you have to give them sixty days notice to vacate. If you engage in self-help (shutting of the water, throwing their clothes on the lawn, changing the locks, etc.) YOU have broken the law.
The way to get them to leave is to serve them proper notice. Your eviction notice has to say you are terminating their right to be on the premises, give a specific date at least sixty days out, and state that you are demanding full possession of your property at that time. It needs to be given to the tenant. Make enough copies and give a copy to each person that you want to leave. Since this is obviously a contentious relationship, when you give the notice to them either get them to sign and date a copy agreeing they received it (they probably won't do that) or have a disinterested party with you when you give it to them. It can be anyone over 18; it would be a good idea to pick someone who would be a good witness in court.
When the sixty days has passed, and they are still there, you will need to file a complaint in Superior Court. You will file a complaint and ask the court to serve a summons on the people. In addition to your complaint, file a declaration requesting monetary damages and eviction costs. The summons will be served by a professional. All of that will cost you some money, which you must request be reimbursed to you in your complaint.
The court will establish a time frame. If your unwanted guests do not respond, you can then ask for a default judgment. Once you have that judgment, you can call the sheriff to have them physically removed. You will have to make provision for them to get any property they own as well.
I don't see any provision in the law that says if someone is 62 they can't be kicked out. It's your property and your decision who lives there. Even if there is, so long as you file before she turns 62 you are fine.
There are no shortcuts that will work, particularly if you've waited 16 years to act. What they want or what you have against them are irrelevant to this. So long as you own the property, you can evict them.
Below is a good summary of the eviction process specific to California.