Like I said, you want diversity of ages. I'm in my late 40s, I work with 20 year olds and 30 year olds and a guy whose even a fair bit older than me.
Hire the best people first, then make your team diverse, both across thinking and across ages and across everything else.
But there's a synergy I've noticed more than once when pairing a 40ish with a 20ish. OR even a 20ish, 30ish and 40ish three person team.
I'm obviously not (and it should be clear in the above message) calling for quotas, but it's a good goal.
Homogenity reduces ingenuity.