OK, so I left home at 17 - went to Uni, lived overseas, then in a flatshare near where I worked. I was 'at home' for about 6 weeks, twice, in that time. I then was fortunate enough to buy with my then fiance, when I was 28. My sister went to Uni, met her now partner, and they moved in together, 4 hours from home, where they had gone to Uni, and got jobs.
But that was almost 20 years ago. It's very different for young people nowadays.
My brother stayed at home until he was 27. He went to a Uni near home, and got a professional training job straight out of Uni. He paid 'digs' to my parents, towards bills and food. He and his partner are now considerably better off than me - she also lived at home until her late 20s.
I think it's never been that unusual for quieter, maybe more studious, less confident, boys, to live at home longer. It doesn't make them weird or unusual. So much depends on the reasons/circumstances. Is this 31 year old a total Mummy's boy, is he studying/training for something, is he saving up for a deposit, is he between jobs/homes? All of these are reasonable reasons for him being at home. Yes, it makes it more awkward if you want to have sexytimes, but I would think he has an agreement with his parents about if he wants to bring ladies home!