How old is too old to use one of these for naps? My LO is 4 months, and will be going to grandma's house while I'm at work. My mom needs something for her to sleep in for naps (she takes about 2 to 3 one hour naps every day). My mom has balance issues due to knee replacements so a pack n play will be very difficult for her to lay her in. I was thinking she could use a rock n play? I just don't want to spend the money on one if she's too old for it!