Years Old
suffix
The kanji and the word are exactly the same. That means they share meanings as well. This is put on the end of numbers to show how old someone is.
さい
The reading is the same as the one you learned with the kanji.
三十歳を過ぎてから、全身がすごく痒いんだよね。
Ever since I turned thirty, my whole body has been itchy.