Having grown-up friends, or as grown-up as you are
Outside a same-age marriage, you could end up spending the evenings with people who use the word "awesome" and think of Debbie Harry as really old.
Same-age marriage means not having to hang out with people who never eat marmalade or buy a newspaper - or shave.
Consideration for your children
Obviously. Children do not want to think of their parents having sex and if you stick to your age group they'll never have to.
Shared experience
The opposite of that is someone who has never smoked on an aeroplane, or washed their hair with one of those hoses you attach to the taps; who has no clue what Asteroids was, or a Joan Collins Cinzano ad, or who Susan Penhaligon is ... Horrible.
Musical harmony
This is a big one. Huge, if your scratched copy of After the Goldrush is still the album you are both most likely to put on at 3am. Can there be anything more nails-on-blackboard than someone who thinks Alexandra Burke was the first person to sing Hallelujah?
Being extremely critical of everything, and being very comfortable with that
Perhaps you think: "Ah, but that's precisely the attraction of a younger person: they are not cynical". Well, yes, if they're your children.
But your other half, banging on about giving Jeremy Corbyn a chance, or the importance of sleeping with a crystal under your pillow, or the advantages of colonic irrigation? Don't think so.
Sex: being on the same page
You can say: "No look, hang on, I'm lying on my specs/toenail clippers/buying a ping pong table on eBay", and it's totally fine because they Get It, and they were probably looking for an excuse to keep reading The Week.
The I-know-what-I-like phase
You're both past all the same stuff at the same stage. It's not that you are no longer open to new experience, just that a lot of experiences are not new to you. You know that camping at festivals is hell. You have tried thrillers (no, Gone Girl is not going to be any different).
You have risen at dawn to see the Buriburi bird, and hung out with rodeo cowboys and driven down route 101. What's the point if you have to do all that stuff all over again because your partner has 30 years of catching up to do?
Same-age couples rock.