And it wouldn't surprise me if she has. Almost certainly, she's thought it.
While I am sure there are some people who are as pure in thought as the driven snow, this is not an uncommon thought people have about their partners, or even themselves for that matter. Most people just wouldn't dare verbalize it. It is just one of those assumed and unspoken truths.
Hell, just the other day, my wife was looking through some old pictures, and found one of herself, very scantily clad, in her 20's, and the trap was set when she wistfully remarked how amazing her boobs and bum were back then. Of course I told her I think she still looks amazing, because in my eyes, she does.
But that gets to the challenge in these types of scenarios...
Objectively speaking, if they were not our partners, and we knew nothing more about them other than a side by side picture of them when they were mid 20's compared to mid 50's, in most cases, the younger picture would be the one most people said was more attractive.
But that's not the case with a long term partner, at least not in the same way with upwards of a lifetime of experience and familiarity with them. The fact that my wife looked better when she was much younger does not diminish in any way how attractive I find her now, nor does it affect my desire for her.