I'm not American. However, my ex wife was 6 years older than me, and my now and forever wife is 5 years younger than me.
I don't know if that means anything, but, that 11 year age difference between them is real. They are literally from different generations in terms of their mentality, expectations, goals, etc.
I imagine a lot of this could be explained by population growth, and physical and social mobility. There are flat out more people around so it's easier to find a partner you don't hate. There are also more people who are socially acceptable and accessible.