According to today's The Unbelievable Truth (Listen Again available in an hour or so), he was twelve, and held the driver up with a toy pea gun.

Of course, I immediately thought of Tolkien's rag-week stunt. The account in Carpenter's biography only has him "capturing" a bus, which I suppose might be a case of stealing an unattended one rather than actually taking it by force, but the latter seems more in the Town vs. Gown spirit.