As in title. To be clear, the question refers to purely physical aspect of witcher training program, not to wheter they can survive the Trial of the Grasses.
For those who don't know, here are some basic information about the program:
This quest shows just how brutal witcher training is. They were forced to constantly train and could not stop even when wounded. Pre-teens who had not even undergone mutations had to repeatedly do fifty push-ups for missing an archery target which they presumably would have done a lot at that age.
Here is Ciri training at the age of twelve. She is flipping across thin beams blindfolded whilst attacking a target and keeping balance, and Geralt even comments that she's slow. She is even able to get off with a backflip.