I think the average US founder age is 40s. But I'm not sure since most statistics focus on successful founders. This link seems to look at all of them.
The average founder of a business is not founding a VC-targeted startup, or even a technology business. There are a lot more landscapers counted in all of these surveys of successful startup businesses than founders who even considered YC.
https://www.zippia.com/founder-jobs/demographics/