No, in this case you should use the counter suffix 　～さい, which is used to indicate "-years old". For the numbers finished in ichi or hachi, the sound changes when you use the number with the counter suffix "sai". Example: 1 sai = いっさい issai 8 sai = はっさい hassai In the example that you gave, the correct way to say 31 years old is さんじゅういっさい = Sanjuu issai desu. I hope this help you!

“Oikutsudesuka” is a question asking your age so you can reply with your age + “sai” which means ~years old. If you want to keep it polite you can add “desu” at the end. I hope this help you.