Ответ:

  1. Oleg and Valera have got a printer. Oleg and Valera’s printer is three years old.
  2. Tamara has got a skateboard. Tamara’s skateboard is one year old.
  3. The boys have got a computer. The boys’ computer is two years old.
  4. The boy has got a watch. The boy’s watch is four years old.
  5. The girl has got a bike. The girl’s bike is two years old.
  6. The girls have got a tent. The girls’ tent is five years old.
  7. The child has got a compass. The child’s compass is six years old.
  8. The children have got a boat. The children’s boat is ten years old.

Правило: для принадлежности одному лицу используется ’s, после существительного во множественном числе на -s — апостроф, а в неправильной форме множественного числа — ’s.