What age is a good age for my boys to get bunk beds?

My boys are 4 and 2. Both are good climbers and have the oldest dream of the boy on top. I feel that he is quite old, but who cares about the small climb up there even if I say its off limits to him (which will only be a little hard to clean all the time). I know someone who lost an eye because she fell off a bunk (while sleeping) and landed in a plastic garbage can broke at age 3. I am nervous about my kids getting hurt, but the climbers are quite resilient and so good. What age should my kids be for bunk beds?