How often do you go to see a dentist?

I carefully brush my teeth with an electric toothbrush, so I haven’t had a tooth decay for years.

"Tooth decay" is an abstract/general idea, so you don't need "a".

Speaking of which, I found out that I don't have noany wisdom tooth four of themteeth.

To talk about your wisdom teeth, you could say "I found out that I don't have any of my four wisdom teeth" or just "I found out that I don't have any wisdom teeth". I think they are both equally natural. I think it's quite rare for the number of an object to come after that object in English.

I think I’ll keep on brushing my teeth so that I’ll be able to enjoy my eating life.

No change suggested

This is grammatically perfect and completely understandable. Just as a suggestion, instead of saying "my eating life", I think more natural alternatives are "so that I'll be able to eat what I want" or "so that I'll be able to enjoy what I eat".

I carefully brush my teeth with an electric toothbrush, so I haven’t had (a) tooth decay for (or: in) years.

Speaking of which, I found out that I have no wisdom toothteeth—I’m missing four of them.

I think I’ll keep on brushing my teeth so that I’ll be able to enjoy my eating for the rest of my life.

I’m not sure what you mean by “my eating life, so I made a guess.