Is it harder to do it the older they get??
I would think it might be easier...we did it at 2.5 with Masyn because she wasn't in a bag and could get out, so risk of falling. But, she couldn't open her bedroom door then, so we emptied the room of everything but the bed, so she couldn't get out anyways!
I think alot depends on personality & sleep habits too, Spencer went to a BGB at 16 months (crib recall, we took the side off of it) and it was super easy because she doesn't move in her sleep and sleeps in the dark. She still doesn't realize she can get out of her bed until I turn the light on!
I would have kept them both in longer if I could have, but for us it wasn't really a big deal.