As a man I don't necessarily believe men don't notice wives over the age of 50. We'd have to define what it is you mean by "notice".
Is it playfully flirting, complementing, being romantic, or sexually pursuing them?
Ironically a lot of women during their youth discourage their men from pursuing them sexually. In other instances some older women have a low libido after going through menopause and lastly some women physically change dramatically over the years. Insecurity may arise when she notices her husband sneaking a peek at younger women or those whom she feels are more attractive than herself.
There is an old saying: "Familiarity breeds contempt" While I do not necessarily believe this to be the case it is still very commonplace for couples in long-term relationships to get "comfortable" or take one another more for granted. They stop trying to "surprise" each other. A lot of women don't make the effort to initiate sex with their husbands. Anytime one feels like (they're) always the one making the effort eventually they will stop.
Men want to feel (desired) by women. I suspect this is one of the reasons why a lot of them frequent strip clubs, watch porn, or hire escorts. In each of those scenarios the women (behave) as though they can't get enough of men and are willing to do anything to please them. This the opposite of how most wives treat their husbands.
"Monogamy becomes boring when couples become lazy."
Many couples seek out ways to rekindle passion in their marriage. The problem is they should have never allowed it to die. Some people believe the less sex there is in a relationship the deeper the love is. However once sex has been discouraged or reduced dramatically in a relationship it becomes "platonic" the equivalent of siblings or roommates with the same last name. The excitement dies.
"It's easier to maintain a fire than it is to reignite a spark!"