1.one year.→i-ssai 2.two years.→ni-sai 3.three years.→san-sai 4.four years.→yon-sai 5.five years.→go-sai 6.six years.→roku-sai 7.seven years.→nana-sai 8.eight years.→ha-ssai 9.nine years.→kyuu-sai 10.ten years.→jyu-ssai 11.eleven years.→jyuui-ssai 12.twelve years.→jyuuni-sai 13.thirteen years.→jyuusan-sai 14.fourteen years.→jyuuyon-sai 15.fifteen years.→jyuugo-sai 16.sixteen years.→jyuuroku-sai 17.seventeen years.→jyuunana-sai 18.eighteen years.→jyuuha-ssai 19.nineteen years.→jyuukyuu-sai 20.twenty years.→hatachi（nijyu-ssai） 21.twenty one years.→nijyuui-ssai 22.twenty two years.→nijyuuni-sai 23.twenty three years.→nijyuusan-sai 24.twenty four years.→nijyuuyon-sai 25.twenty five years.→nijyuugo-sai ・・・

For example, I'm twenty years old. 例えば、私は 20 歳です。