Dumbledore and Snape naturally understood that Yinsen meant his apparent age was still too young, below the standard for entering the Triwizard Tournament.
But Harry Potter did not meet the requirements either. When Voldemort's lot had forcibly shoved Harry Potter into the competition, they had apparently taken into account the iron rule that "the Goblet of Fire's decision cannot be changed."
If the Death Eaters could do that, why couldn't Dumbledore?
This had long ceased to be a normal competition. According to the Death Eaters' arrangement, Harry would practically be handed the victory. This was nothing more than a staged performance, and one following a script written by the enemy.
As their sworn enemy, the proper course was to refuse to follow their script and cause them trouble while they were at it.
"So you just need me to make sure Harry comes to no harm, right?"
"Yes."
Dumbledore's main reason for having Yinsen represent Hogwarts was safety.
First, Hogwarts' representative would certainly be drawn into this conspiracy. There was no sufficient guarantee of safety, and even with advance warnings, accidents could still happen.
Second, Harry was Voldemort's primary target, and under the current arrangement, Harry Potter would almost certainly face Voldemort head-on.
That meant Harry Potter would be placed in the most dangerous environment possible. Under such circumstances, Harry Potter could very well lose his life.
During internal Order of the Phoenix meetings, Sirius had strongly opposed putting Harry Potter in such a dangerous situation.
Recently, Sirius had been trying to persuade Harry Potter to move out and live with him. Aside from being Harry's godfather, he wanted to protect Harry Potter at close range and deny Voldemort any opportunity.
He already felt that his protection had not been thorough enough. How could he possibly stand by and send