Really does depend on the children. We had ds in the top bunk before he was 3. We 'trained' him to climb up and down safely and he knew to wait for us. Once he was confident he was fine getting himself up at night etc. I don't recall any real issues with dd1 climbing up. They both understood that they weren't to play up there.

Dd2 at 19 months confidently climbs up there now! Tho we generally try to stop her heh.

Our kids don't seem to be silly with it. They're all very agile and fairly sensible. But I have seen some young kids with no sense of where they are .. Fall off the edge of couches etc. so I think it really depends how you think your kids would be with it. I dunno.. Maybe cos we did it so young, there was less novelty/silliness.