My older two boys went into a top bunk aged 2 (literally turned two that month) and 3. We didn't want the 3yo in the top bunk but a tiny room meant there was no space for two beds otherwise. The 2yo didn't want to sleep without his brother so climbed up when we put them to bed and they slept in the same top bunk for almost a year. It really depends on the child though, and had it not happened like that I would have probably said no to a top bunk or cabin bed until nearer 4, more for the risk of falling whilst climbing the ladder into it that anything else.