"9岁" 用英语说为 "nine years old"。
其中 "nine" 是数字 "9" 的英文表达，"years" 是 "年"的英文表达， "old" 是 "年龄"的英文表达。
例句：
"My son is nine years old." (我的儿子9岁了。)
"She looks very young for her age, but she is actually nine years old." (她看上去很年轻，但实际上她已经9岁了。)
"He will be nine years old next month." (下个月他将会9岁。)