I have a question about when implants are needed.

I've heard that you should consider implants if your teeth are very loose or difficult to treat due to cavities. I'm curious about the criteria.
I have heard that it is best to save natural teeth as much as possible, but I would like to know at what stage of condition extraction is necessary.
I am wondering if dental implants are possible even at an older age.
I would also like to know the situations in which a bone graft is necessary.
I am also curious about how much the lifespan of an implant varies depending on care.
I am wondering which is the better choice: holding onto my teeth for a long time or getting implants quickly.