Venture capital is a hits business, and if you want to invest in an entrepreneur there is benefit towards getting them the first time. Marc Benioff has 13 years of Oracle experience when he started Salesforce, and he already had plenty of access to capital and expertise.
It makes sense to me that older (successful) founders likely leverage their own experience and network to a greater extent, but this isn't necessarily what seed stage investors are looking for. I think investors are more in the game of betting on people/teams, injecting their own experience and network as leverage. Older founders have less need, therefore less leverage- and older founders without these support networks are poorly positioned anyway.
Theres a big difference between running a successful small business and architecting a tech startup.