Candice Wing says

I’ve met a good many mature men looking for affairs and divorced men looking for a second wife. None of them have said – “oh dear me, my wife is old and fat and thus unattractive and therefore I feel compelled to seek a younger and therefore more attractive option”.

Source: Candidly Candice

She goes on to list real reasons men have told her for separating from their partners

• Wife does not want to have sex with me or wife does not want to have enough sex with me.
• Wife does not like me and does not have sex with me.
• We are not compatible and I am looking for more than just boring sex.
• Wife is not affectionate.
• Wife is boring in bed and generally boring.
• Wife is a cranky harpy.
• Wife is lazy and boring with poor grooming and presentation.
• I (or wife) want to divorce.

You can read the whole thing yourself, and if you do you'll get her simple one-paragraph explanation of why the vast majority of men remain perfectly attached to their partners.

Candice has been writing a lot about her own experiences sex, love, and aging. This is another great post along those lines.

While, sure, some people (not just men) really do lose interest specifically over their partner's looks, it happens at any age. And if it happens at any age then emphasizing one age over another is just stereotype reinforcement.

Meanwhile the other reasons you list are much more plausible, particularly for very long-term relationships. Although, hmm, now that I'm thinking about it even that shows up more predictably at certain points in a relationship than at certain ages. For instance I seem to recall there's a spike in divorce rates at the 21-22 year mark whether the couple marries in their late teens or mid 40s. And if you just think about it for a minute, if some people in their 40s find their flames going out while others in their 40s find themselves igniting, then age probably isn't the cut-off factor young people, hack novelists, and pop social scientists keep claiming it is.

Either way I agree with Candice that it's way more complicated than the popular but too-pat stories about husbands leaving because their partners "lose their looks" post-menopause. In fact it's so complicated it might not be happening for specific age-related reasons at all.