Technically にじゅっさい is not wrong, so feel free to use it. Just be aware though, that はたち also means the same thing. Just like 20 years old, 30 years old, 40, 50 and so on are formed in the same way. Take the number for 30 and add sai ( さい ). No biggie. See you next time! Tags: Japanese age Keep learning Japanese with us!. When you are 20 years old, it is unlikely that you have faced many detractors to rob you of your dream. Therefore, believe in it and never stop. Live the life you desire, not the one that your parents or other people suggest. Keep an open mind and make a plan about how you can achieve your dreams.