How old was Wu Daozi when he began to concentrate on painting?

Later, Wu Daozi served as Qiu Wei in Yanzhou. The complicated official duties made Wu Daozi unable to paint with peace of mind, so he decided to abandon his official position and run away. At the age of 20, Wu Daozi came to Luoyang, the eastern capital, and devoted himself to painting. He studied under Zhang Xu and He, and his classmates had a famous sculptor Yang, who was later known as "shaping saints". Because of his obsession with painting, Wu Daozi gave up calligraphy and devoted himself to painting.