Is L a climber? I think my concern at this point would be the younger one getting up on the ladder...
A has had one for his room since he was 3.5 or so (about 3y now). He loves it.
We'll probably end up upgrading his bed and giving the twins the Kura, with one mattress up and the other on the floor -- but not for several years yet, since they are monkeys and would climb and fall, no question. (I don't think I'd be comfortable with a bunk bed for either of them until they're at least 6, at this rate.)
mommy to three boys: A, 3/04
and identical twins B and D, arrived 9/08