I'm 54, poor reflexes, slightly blurry sight. I can see well enough, there is a noticeable difference when I wear my reading glasses, but they are uncomfortable, so I only use them when I have to. I don't think it affected this, but I'm a very below average person on physical capability. Not disabled, but questionable. I unlocked the last thing in the tree the round before I got level 5 on the right. The balance was perfect for someone like me. You should probably make it a bit harder.
I spent most my game trying to get gems, not gold and mostly bought what was cheap. I kept wondering when am I getting chain lightning? Nice demo.