@Samantha_Rae grey does not typically respond to tooth whiteners like Crest strips and other peroxide products. Yellow stains are what respond really well. Yellowing is caused by the things you mentiond above; dark soda, tea, coffee. I don’t know what causes greying later in life. Young children who take tetracycline, or if their mother took it while the child was in utero, get grey teeth as a side effect. They typically have to have their teeth capped, or some other method to bring them back to white, they cannot utilize the teeth whiteners that we are accustomed to seeing in the stores.
I wish I knew the answer, knew more about greying and what you are experiencing. I can tell you this, your dentist should know if your type of staining will respond to whitening gels and liquids.
Meanwhile it can’t hurt to try the baking soda and some of the other suggestions.
I wish you luck.