The prettiest, most grown-up bed I've ever slept in was a twin. I was 21 and not really a pink person, but I loved that bed upholstered in a rich rose linen. While twin beds otherwise make me think of little kids, the moral of the pink bed story is that a well-styled twin can maximize space without looking juvenile.
Twin beds work well for especially small or awkward bedrooms, guest rooms, and of course children's bedrooms (especially ones that are shared). Sure, a twin bed won't work well if cuddling is your priority, but it will free up lots of space for storage, work space, or, in truly tight quarters, just room to get around.
MORE TWIN BEDS ON APARTMENT THERAPY:
(Images: 1. Vintage House 2. Loft & Cottage 3. Two Ellie 4. Barry Calhoun for Style at Home magazine via Poppytalk 5. Haven in Paris via A Cup of Jo 6. Little Fine Day 7. Gregory Han for Apartment Therapy 8. Scuba's Open House)