Well, there are several causes for this, but they boil down to two:
Him. He may have a low libido. I suggest reading up on the reasons for this . Depression is a real common cause, as are sleep issues and low testosterone. Many can be fixed or treated by a doctor, diet, exercise, etc.
A secondary cause could be his interests could be directed elsewhere; namely porn or another woman. Getting his release through porn is unnatural to do as a primary outlet. Also, his change in sexuality is an indicator as well as cheating. If you think those two are not it, more than likely it is the health reasons. I’d venture to guess if he is middle-aged, it is health + porn.
That being said, has the quality of your sex changed for the better over the years? When they say, “spice it up”, there is something to that. Do you have a D/s type sex life when it is on? Do you sext with him? Engage him in new ways. Maybe he wants to explore some shit he is not comfortable with talking about, like bondage or DD/LG, but doesn’t know it or know how to do it.
Bottom line is – you guys need to talk.
You. This part may sting a bit, but only you will know. Try and understand men, from a biological standpoint. Men are visual creatures and we are turned on in different ways than women. We watch porn, but don’t care about reading stories or erotica, because it doesn’t do it for us.
When a woman tends to let herself go, guys tend to back away. So I would ask, do you take care of yourself? You should, for yourself, but I can’t imagine he will increase his pursuit in you if you don’t put effort into the physical representation he sees. I know it sucks. I know it’s fucked up. But it is in our DNA.
Told you – that part will sting – but in the long run, it will help to know. And last, No, age is not really a factor. A hot 55 year old with a sexy woman’s body who goes to the gym days a week and eat right will get attacked by her husband just the same as she did when she was 25. We don’t care about wrinkles or things you care about. We don’t even care about weight as much as you. A little extra weight; no big deal. But you have to find that visual zone for him, just like he needs to find that zone for you that turns you one when he “gets off his ass, engages you, listens to you, works hard for you, takes care of you, eats your pussy, makes you cum, et cetera.”