They tend to say "to age 6", so my working assumption has been to age 5ish.
DS has one because (a) we got it cheaply on eBay and (b) that way we can fit his toddler bed plus a double futon in his room (if we had the futon down, admittedly you couldn't actually walk anywhere in the room, but technically it fits) which may prove useful at some point (c) I know that eventually DS would love, love love an elevated bed, and this way we can think about that when he's old enough rather than pursing my lips and saying "you've got a perfectly good single bed already, I don't know... kids today...".