You can wait until you are under way. Check it out the first day at sea to see if it is something you want to do. I would think that you would need to be able to pull yourself up the face of the wall, but I do not think they have a weight limitation for it. You are harnessed with a safety line just in case you fall.
The staff really works with you (especially kids) to make sure you get to the top. I was on one cruise where this kid (around 10 or so) wasn't very coordinated. Hed tried and tried to get to the top. On the last day at sea, one of the counselors coasched him all the way to the top and the kid was so heppy he couldn't stand it.
I have never climbed it myself, but it looks like fun.
Let us know if you make it to the top.