Flippant answer: in the U.S., in your twenties, you have no spare space, and visiting friends sleep on your couch. In your forties, you have a guest bedroom, and visiting friends stay at a hotel.

Possibly more accurate answer: it depends on what kind of housing people live in, if they have kids, and if they work at home. Most residential houses were built for couples with children, so if someone owns a house and is single and/or childless, they likely have spare bedrooms that serve as a home offices, hobby spaces, or guest bedrooms. People living in apartments usually don't pay for more space than required for their daily needs.