In some of South America your age is based on your nearest birthday, not your most recent past one. I don't know if Argentina follows this practice, but if it does, and you were born at least 20 years and 6 months ago, you'd be considered 21 years old and therefore OK.

(I seem to recall a flap at a world beauty pageant over this several years ago. It had a minimum age of 18, with the concept of "18" not further defined in the rules. One of the South American contestants was 17 years and 6+ months. She was considered 18 in her home country, but not by the world organizing body. I don't recall how it ended.)