Conversation so far:
when did Donnie Yen start martial arts?
He learned Tai Chi from a young age under his mother's tutelage.
who trained him?
his mother's.
how long did he train?
CANNOTANSWER.
Are there any other interesting aspects about this article?
His exposure to mixed martial arts (MMA) was heightened when he went back to the United States from 2000 to 2003.

Question: how was it heightened?
Self-contained rewrite: how was Yen's exposure to MMA heightened?