I am just wondering what age you put your kids into a bunk bed (or would you put your child into a bunk bed)?

We will be space deprived once number 3 comes along and we've got a bunk already so I was toying with the idea of putting DD and DS in the same room.

DD will be 3.5 yrs (top bunk) and DS 18mths (bottom bunk) when they'd be roomed together. I was thinking of buying a trundle bed but DH seems to really trust DD's climbing ability. And yes, she has been able to climb ladders for about 6 mths (at the park etc), but I'm not entirely sold on the idea. Maybe its a case of giving it a try?