YOU SAID:

I'll turn 18 in one week

INTO JAPANESE

私は1週間で18歳になるでしょう

BACK INTO ENGLISH

I will be 18 years old in a week

INTO JAPANESE

私は週に18歳になるでしょう

BACK INTO ENGLISH

I will be 18 years old on weekdays

INTO JAPANESE

私は平日に18歳になるでしょう

BACK INTO ENGLISH

I will be 18 years old on weekdays