It all depends on your circumstances. In my case for example, I'd lived all my life living with my parents until their deaths at ages 86 and 87, by circumstances of my occupation and marriage in my birthplace. Although we had separate homes when I got married when I was 29, we lived as immediate neighbors, that's living together just the same.
If you're still single at 40, I don't see any problem if you continue living under the same roof with your parents. Living arrangements must be arrived at with utmost respect to them especially when they're growing old. I guess you know what I mean?