网上的东西不可靠,要看字典.在某人多少岁时(主要是10多岁,20多岁,30多岁等)的句型是:in one's +20,30等的复数,这是固定的.没有in his fifth,这是错误的.在他8岁,应该是when he is 8, 在他18岁when he is 18. 如果表示“在.岁时”,就是at the age of, 如:8岁时,at the age of 8. 18岁时,at the age of 18
1.at the age of+某人岁数e.g:At the age of 18,LiMing learned how to drive cars.在李明18岁时,他学会了如何开车。2.in one's+数字基数词复数e.g:in his twenties在他20岁的时候in Wanghua's sixteens在王华16岁的时候