Communication is the corner stone to everything in a relationship - including a good sex life. Maybe his age is getting to him, maybe he has some serious stress happening at work or in other areas? If there isn't a physical explanation then it's mental for sure. Relationships have ups and downs when it comes to sex, things get in the way. Sit him down and talk to him about it, maybe he will offer something up as to why. And you can work from there.
I do agree with jml1986 talking is the best, and trying out new different things that you have not done before, i don't just mean sexual, anything, suprise him with dinnear, going to the cinema that kinda thing, maybe the age is getting to him, his feeling insecure thinking that you could leave him and go off with a younger guy??
Communication is the best advice I can give. There maybe a physical reason why he is no longer able/willing to be sexually intement with you. My husband is older then me and we have had our ups and downs with our sex life, but I recently had a hysterectomy and now are sex life is off the charts. What I am getting at is, I had no idea that the reason I was not that interested in sex was because my body was not allowing me to. So please talk it out and work on a solution together.