Does Natsu have an known actual age?
Well, in chapter 436 it is revealed that, ...


   Natsu is about 400 years old. Give a few years between the flashback  and his birth and take seven years during the time-skip. He was born 400 years ago as the younger brother of Zeref. He died soon thereafter, but his body was kept intact by Zeref during all those years. The body Natsu is using in the current time-line is the exact same as that of Zeref's younger brother. More can be read at this related question about how this all came to be.