S.H.
I would say that depends on your oldest, My 7 yr old loved getting a bunk bed when she was 4, my now 4 yr old is very nervous getting up and down and it wouldnt make for a good situation, she will go up and read with her sister before bed and I usually have to help her down.... I would not put her on the top bunk full time. Maybe take her shopping and let her climb up on a couple and see if she likes them.
HTH
S.