Worldwide average between brides and grooms is 3 years.
This happens simply because genetically, females seek out males who can provide for them the most.
In traditional hunter-gatherer societies males have their peak access to resources in their late 20's when status and physical health are high, or later in life when accumulated experience, skill, wisdom and social status give advantages.
Although recent studies show that females may have evolved a preference for men who show signs of high parental investment. So if you're great with kids, it's likely women would naturally like you more as well.
because men mature at a different rate than women. so when girls are 16 and likely done growing, guys who are our age still have another growth spirt possibly and haven't grown facial hair. so girls tend to look at older guys who are at the same stage of development as them.
I'm speaking only for myself here, older. Younger guys have some growing up to do and I'll just get older waiting for them to grow up! Unless I find a guy who meshes with my level of maturity and is younger/same age, I tend to go older.
That's the basic instinct of females being attracted to males. The male is supposed to be dominant, the provider and the disciplinarian, being older and more established makes a man appear to be those things than say, a high school sophmore.
Of course there's also the possibility that that those girls are just using the older guys to buy things for them.