At 20 months we put DS into a bed, but we got the reversible bunk from IKEA, its called a KURA
Much Cheaper - we started at floor level and then turned it over to normal bed height, and will turn it over again for a small bunk in a year or two, very flexible so the bed can grown with the child, and DS thinks is the coolest thing ever has it has a tent roof!

A few piccies
94/365 – Big Boys Bed « Graetzfamily's Blog (with mattress at floor level)
315/365 – Bedroom Overhaul « Graetzfamily's Blog (bed flipped to normal height)
You can flip it again so the bed is about a metre or so off the ground, if you have a look at it on the IKEA website you'll get what I mean