I don't know where you're getting your data and I've forgotten where I got mine but I believe most companies are started by founders who are 50+ years old. Maybe my data includes 60 year olds, etc... As well.
But, we both agree it is not the 20 something demographic. 2 unsoirced opinions on the internet - that agree - have to be correct.
To be fair the largest demographic of startup founders is in the 30-39 age bracket. While successful twenty-somethings with big exits is something the press likes to cover, they are very much outliers.