Not sure what you mean exactly by assembled, the walls of it are collapsable so it comes in the box collapsed. However the walls and bottom are all one piece so all you have to do is click the sides to lock them in place and if you use the bassinet part hook the tabs over the side. Insert the support bars in bottom of bassinet and throw in the mattress part. It took me about 15 minutes the first time I set it up. It is very easy. No tools required.
There is a mat (pictured as the green section in the photo) that unfolds on top that has a slight padding on it. It is like a mattress but not as thick. I never felt the need to put a mattress down because of the slight cushion it provided without being too hard or too soft. As your baby gets bigger/more mobile you can lower the mat to the bottom of the pack'n play to ensure safety.
I would say birth to about 6 months for the raised bassinet, and then probably up to a year for the play pen. I have been using it as a crib for my 5 month old since she grew out of her little bassinet at 2.5 months.