[vostok]

In the US typical average age at matriculation is around 27-28 at top business schools and they have formal/informal work experience requirements.

People who get in straight out of college are typically doing 2+2 or similar programs although I know that submatriculation is technically possible at Wharton.

To your comment about people getting in with no undergraduate studies, this is extremely uncommon. The exceptions tend to be people like Blake Gottesman who went on to be very successful.