Dr. Pepper Schwartz: I don't think it's age-related. She is way past the early years of menopause that sometimes jerk women's sexual desire around and affect sexual habits. And there is no biological reason for a woman who has enjoyed a sexual act to stop liking it (except perhaps if it is painful, which sometimes happens during intercourse for women who no longer lubricate easily and are not using an artificial lubricant). So, while it is hard for me to really know from this little bit of information, my guess is that there is a relationship issue that is presenting as a sexual issue.
The two of you need to talk longer and deeper about what's going on. And you need to be more observant and see how she is acting in other parts of the relationship: Is she as affectionate as has been in the near past? Is she depressed? Does she snap at you more or give you the same amount of time? Take a look at the whole relationship and see if this fits into a more general pattern. If it does, see a therapist before the relationship gets worse.
If there are no signs of change and she is still affectionate, it is possible there is some vaginal problem, such as vulvodynia, a condition characterized by pain, burning or irritation around the opening of the vagina. However, any of those problems would have made intercourse an issue as well, so I am inclined to think that's not the issue.
But start with the simplest approach first: Ask why oral sex is no longer satisfying to her, and ask in-depth questions. For example: Is there something that has changed that is hurting you?
Is there something different you need in technique? What do you think would feel good if we tried it? Perhaps getting more detail will give you the answers you need.
Photo by escapologist.