If you were born in 2004, you will turn 17 years old in 2021
To work out how old someone will be in a certain year you can use a very simple formula. The formula is to subtract the year that the person was born from the year that you're looking to find their age in. This gives you the age that that person will turn that year.
This means that before their birthday in the target year they will be this age minus one, however, once they have had their birthday they will be that age.