We got bunks when DS1 was 3 and DS2 was 2, expecting DS1 to be in the top bunk (there was no room to put the beds side by side). What happened was the boys did not want to be separated and so they both slept in the top bunk for months. Not how I would have planned it but it worked for us and gave us the bottom bunk as a spare for when my mum stayed over.