"Ninth birthday" 用英语说就是 "ninth birthday"。
"Ninth" 是形容词，意思是 "第九个"，"birthday" 是名词，意思是 "生日"。所以 "ninth birthday" 就是 "第九个生日"。
一些例句如下：
"I can't bepeve it's already your ninth birthday."
(我不敢相信你已经九岁了。)
"Happy ninth birthday, my dear!"
(亲爱的，祝你九岁生日快乐!)
"Let's make this ninth birthday one to remember!"
(让我们让这个第九个生日成为难忘的一天!)