The Trial of Long Grasses is administered in youth because the body is more receptive to change. I believe the insinuation is its basically an illusion to puberty and how the body changes then.

It doesn't make sense for Ciri to do it regardless of her powers because she can't live through it as an adult.

And if they say "oh well she lived because of her powers" then why would she need to do it at all if she still has them? Her Elder Blood powers are away and beyond any being left on their plane of existence. She doesn't need some comparatively hack job, bootleg attempt to become stronger.

In my mind, it's akin to a Primarch from WH40k, chopping his limbs off and jumping in a Dreadnought armor to "enhance" himself.