Sexual

If it is a situation you are unhappy with, you could try to change her mind. Sometimes, when women pass their "sexpiration date" [as I heard someone call it once :) ], and there is no biological/medical reason why this is the case, it isn't necessarily due to age. Rather, it is sometimes due to emotional/interpersonal dynamics in the marriage that discourage them from wanting to have a physical relationship. Sometimes it also due to frequent childbirth and not wanting to conceive again (which, I guess, probalby isn't a consideration in an older age). 

Both men and women can vary throughout their lives in terms of how interested they are in having a physical relationship, and sometimes there are hormonal or medical reasons that affect this, including but not limited to health conditions and the side effects of medications, as well as lifestyle factors such as stress and exhaustion. So this may also be a factor. 

[Of course every situation is different, I am just putting these out there as general possibilities]

If she is quite certain that she would like to close this chapter of her life, well, your formal Islamic options are the standard options. Either you could just accept it  (this is usually what society encourages women to do in this situation), or you can take another wife. Or, perhaps she might be amenable to other forms of physical intimacy, if that works for you.

Anyway, life is rarely easily and ideal, and the interaction between the two genders is also rarely ideal and has its own lifelong challenges. Perhaps prayer to find the best solution is a good start.