> the smartest people happen to go to Stanford
There's no way this is even possible, let alone true.
This might be a hot take but if you're under the age of 25, it's likely that you know very few things total. And the things you do know, you're just not going to have a master level of understanding of in your 20s yet nor the drive to focus on it the way it needs, especially with other social pressures at play. What is extremely high in your 20s is your ambition - your loftiness of ideas and goals and your expectations of yourself - these can land you exceptional jobs, opportunities, and get you to a great starting point, but that's not the same as intelligence and ability.
For manual labor work, there's a definite peak - college age and the years just after. But for knowledge work, math, programming, business, politics, etc. being older is almost always an asset - you get better at things, you get smarter, your networks grow, you become an adult among the children and are far more aware of timing and can predict and speak on things with more accuracy. You simply know more.
Besides that, I don't think raw intelligence is selection criteria for a good founder anyway. It has a lot more to do with your particular situation - how well you're situated to pull it off, which can mean many things. In terms of personal qualities it probably has more to do with determination and obsession (work ethic) than intelligence, especially these days where information is so cheap and available.