When asking about age, you have two choices (that I know of):
なんさいですか。(nan-sai desu ka) and おいくつですか。(oikutsu desu ka)
You answer by saying (age)さいです。おいくつ is more formal than なんさい.
Most ages are said the same way as you would say the number + sai, but there are a couple of exceptions. For the age 8, you don’t say hachi-sai, you say hassai, for 1 you say issai, and for 10, you say jussai. 20 is an exception as well; you can either say ni-jussai or hatachi.
These exceptions carry over to any ages ending with 1,8, and 10.
For example, 28 would be nijuu-hassai and 40 would be yon-jussai.