“…WordPress network must use sub-domains.”

When enabling a site for multisite, I got this message when setting up the network:

Because your install is over 1 month old, the sites in your WordPress network must use sub-domains.

What does that mean?

Is this just a matter of still being in beta?

Could I avoid this (and the option to put the network in directories) by uninstalling WP and reinstalling it? If I were to do that, would the new install find the existing posts/content in the database?

Thanks!