It is often heard from the middle-aged couples on the apathetic nature of their sexual partner, yeah as the age goes up the interest to physical intimacy may lack in many of us. But it is not only because we lose the interest to explore the partner's body, nor it is due to the multiple partner relations. It may be due to sexual fluidity.
(this content mentioned below in a paragraph is purely from what I read while going through some popups and does not involve my explanation at all!!!!!!!!!)
Sexual fluidity refers to the idea that a person's sexual preferences and attractions can change over time based on their experiences and relationships. Unlike the more static notions of sexual orientation, sexual fluidity acknowledges that people’s desires and attractions can be dynamic and not necessarily fixed. 
So, the question is how can we deduce this apathy? Yes, it is a fact that we can't completely erase such a lack of interest in one go. But some of the following remedies may help you or your partner to overcome it.
Proper communication: If you open up your feelings with your partner about what you desire, what you long for then the sentimental approach might have some results.
Spend some quality time together outside of the bedroom to strengthen your emotional connection. Engaging in activities you both enjoy can reignite feelings of closeness and attraction. It will be a fire if you keep youth within you- innovative, explorative and active.
Healthy measure: if you do regular yoga/exercise it may help you to have a happy sexual life. It is not just through exercise but by having some diet plans, safe practices you can have a healthy body and as always, a healthy body possess a healthy mind! If you are bold about your physique, it will increase your interest to engage in physical intimacy as you are confident on you. 
Introduce novelty to your sex life, within the bedroom nothing should be hided. Hiding things=generating doubts=unhealthy relations.
Dr Dear
(this content mentioned below in a paragraph is purely from what I read while going through some popups and does not involve my explanation at all!!!!!!!!!)
Sexual fluidity refers to the idea that a person's sexual preferences and attractions can change over time based on their experiences and relationships. Unlike the more static notions of sexual orientation, sexual fluidity acknowledges that people’s desires and attractions can be dynamic and not necessarily fixed. 
So, the question is how can we deduce this apathy? Yes, it is a fact that we can't completely erase such a lack of interest in one go. But some of the following remedies may help you or your partner to overcome it.
Proper communication: If you open up your feelings with your partner about what you desire, what you long for then the sentimental approach might have some results.
Spend some quality time together outside of the bedroom to strengthen your emotional connection. Engaging in activities you both enjoy can reignite feelings of closeness and attraction. It will be a fire if you keep youth within you- innovative, explorative and active.
Healthy measure: if you do regular yoga/exercise it may help you to have a happy sexual life. It is not just through exercise but by having some diet plans, safe practices you can have a healthy body and as always, a healthy body possess a healthy mind! If you are bold about your physique, it will increase your interest to engage in physical intimacy as you are confident on you. 
Introduce novelty to your sex life, within the bedroom nothing should be hided. Hiding things=generating doubts=unhealthy relations.
Dr Dear
