> in general we celebrate our nth birthday after completing n years of life. Well it's literally like you say: your friend will "celebrate his 30th birthday af

> in general we celebrate our nth birthday after completing n years of life. Well it's literally like you say: your friend will "celebrate his 30th birthday after completing 30 years of life". If the 30th birthday happens after 30 years of life, then the 30th year of life happens before the 30th birthday.