My teeth are getting yellower and I don't like the look of them. What is the cause?

As I get older, my teeth are getting more and more yellow, and I think it's getting worse.

I don't smoke, but... could it be because of the caffeine in coffee? I heard somewhere before that caffeine stains teeth yellow, so I think I should consider whitening my teeth.

I asked the dentist I go to and they told me not to get teeth whitening done because it might make my teeth sensitive later on. But since my face is on the white side, the yellowness is more noticeable and I really don't like the look of it.

I've never used whitening toothpaste or anything like that, but I wonder if it's effective... Or is there something like tape that you stick on and then remove? Is it effective?