I ONLY date women 10+ years younger than me. Though I do look young for my age and am in great shape with around 10% body fat. I run 30+ miles a week. Most people guess me to be around 30-32, and that's with a beard. The last girl I dated was 35. Before that was 23. Before that was 24, 33, 31, and 34.
I have no age requirements. If she is 40, has a cute face, is slender and in good shape, and has generally held up as well as I have, I'd be all over it. Trouble is, you almost NEVER see that and if you do, she isn't single, or if she is, she is completely toxic. The younger you go, the better the chance she is single, is fit, and isn't toxic.