Thoughts of Duke Su for consideration
He was old enough to understand there was something bad/suspicious about his father's death. He then went to beat the Deng Wen Drum and his grandfather??? carried him away.
At his father's funeral, he looked through the screen with the young Emperor and the Emperor asked him whether he was as suspicious as he was. Duke Su said, yes.
I'm going to place them at around ages 11-12 here, still considered young enough for XH to be confined by his grandfather for five years after that (to protect him from being killed). He was constantly ill (when he learned how to read pulse) and spent his time watching operas, thus his love for acting. So five years after that is 17.
Then we see the scene of the two young men, of the Emperor giving XH orders to investigate and his trust. They shook hands their special way. By the time the drama started, eight years had past and XH thus was 25.
TADA! What do you guys think?