We really cannot tell. In appearance he is 18, not counting the 7 years. However there are many theories that he is older. We know for a fact that Natsu and Gajeel are at least over 80 years old, in the fighting festival arc neither of them could get out of Frieds barrier. There are some theories that suggest that Natsu is Igneel which would put him maybe over 100 years of age. So just know that in appearance Natsu is 18 but we are unsure of his real age, except for the fact that he is over 80 years old.
