My eldest was nearly 5 i think when she went into the top bunk. Youngest was nearly 3.
At first the rule was that she didn't climb up or down without me or her dad there, but it soon became obvious that they were both quite capable of getting up and down,on their own (not nesercerily using the ladders either).
LOL puff - we've got a 3 bed house and our two boys are in a room together with the 3rd bedroom being an office. They started out in seperate rooms but the 'office' is so cold that we had to move DS1 into his brother's room - as it happens they LOVE it and I wish we'd done it earlier.
Incidently we were talking to day about getting a bunk bed when DS2 comes out of his cot - which will probably be later this year, when DS1 will be almost 5
please warn your children of the dangers though. ds1 and ds2 (9 and 5) both climbed up on ds1 midi bed, messing about tugging on either end of a pillow and hey presto ds2 has backflipped over the end resulting in concussion and a 5 hour wait in casualty
That'll teach me to sit downstairs drinking coffee of a morning