Good Morning S., Our boys were 5-7 when we put the bunk beds together. They were the type that each could be placed on the floor and stacked later. They were old enough then to know how to get down and we always had a good night light to see by.
Son and Daughter law have the same type of beds you described, our oldest Gr son 7, sleeps on top youngest 3, on the bottom. Our grand daughter sleeps on the bottom of her's She is almost 9, and when she has a girlfriend or cousin stay they make her mattress bigger.
Or other son & dl are wanting to get BB's also, and have the oldest 3, get used to it. Right now I don't think he is ready for the top as he gets up several times to go potty and waste time going to bed now. This has just started the last few months. Used to put him to bed, say prayers and kiss's and he was out for the night. Their baby is only 6 months, so there is plenty of time before they a roomies.
My Brother (5) and I (9) used to take turns on top, and we always put our feet on the top bed box springs or frame and lift to see who could make the other fall off the fastest. KIDS, whacha gonna do wiff um! :) We both lived.