You only add “sai” after a number when you tell someone your age, but in the case of 20 years old, it is more common to say “hatachi,” rather than “nijussai.” Asking an adult their age is seen as being rude, especially if you’re speaking to a woman. Many people are hesitant to honestly say their age. This is probably the reason that the vague terms “arasa-” (around thirty) and “arafo-” (around forty) have caught on. On the other hand, for old people, words such as “nennpaisha” (elderly person) and “shinia” (senior) are more often used to lessen the impression that they are past it.