Funny, I know several really nice women who are markedly younger than me, and I too hit the gym and stay in good shape for a guy my age.
And yet, I'd just feel funny dating someone who was young enough to be my daughter.
However, that feeling is strictly personal, and I'm not saying it's off-putting or wrong for anyone else. It just feels odd to me.
Also, for me, the age of children she may have would be a factor. Several of the women I alluded to above have kids who are quite young. I wouldn't want to go back to that. I feel like I did all that and that I really don't want to raise kids anymore.
But again, that's personal, and I think it's fine if an older spouse wants to take on the responsibility of younger kids again.