At what age did Ludwig van Beethoven start his career?

Beethoven started his musical training, primarily because of the influence of his Grandfather (also Ludwig), who was kapellmeister in Bonn, at the age of 4 or 5. He had however, already demostrated a high level of aptitude before this and his ne'er do well father seized upon this as an opportunity to commence the boys training, ill equipped as he was for the task. His father (Johann)treated the young boy harshly to the point of cruelty and violence appears to have been the main driving force behind the young genius' development until Johann's limited knowledge was exceeded, whereafter the assistance of one Tobias Pffeifer was brought in (apparently one of Johann's drinking buddies).