Hi Chocolate!
I am not sure why it says that. . .it certainly can be used for a whole long time! The sites also says that you can use the upper two levels up to 30 pounds and the lower level up to 50, so maybe they are talking about just when you can leave the baby alone in the co-sleeping configuration. But even then, there is a lip of two or three inches on the edge that is attached to the bed, so the baby couldn't roll out no matter what. Crawl out when they are pretty coordinated and strong maybe, but not roll. Maybe that was one of their VERY American 'don't sue me' kinds of disclaimers!
You have to use your head in that you have to lower the level when the baby starts to pull up, etc., and if the baby is crawling, you have to raise the side if you are not in the bed (for naps, etc.) but that is not hard to do. I just undo the strap that secures it to the bed on one side (not both), pivot it away from the bed and raise the side and there you go. But even then, it was a pretty long time before Matthew OR James got that coordinated. . .WAY past six months. But for co-sleeping, you just have to make sure that the level of the mattress matches their ability so they don't pull up and topple over the sides if you are asleep.
I don't know anyone who has one who has used it for less than a year. I just had to put mine down to the lowest level, but it came with leg extensions and you can buy more, so I raise it up a bit with that and though James is lower than the bed a bit, he is still within easy touching distance, and when he wants to feed, he pulls up and tickles me and I pull him into my bed and then he goes right back to sleep.
And there are other uses, too. . .it's as deep in the lowest position as a travel cot, so you can use it for that, which I do and love because the boy's were familiar with it from sleeping in it each night and the sight and sound and smell was 'home' to them wherever we go, so they just go right to sleep. Just before James was born, when I had to contain Matthew, I also used it as a play pen, which Matthew tried to destroy by jumping in it incessantly, so it definitely has other uses, too.
So, I think you can use it for a long, long time. And I still think I should get commission from the company!!
Chelle