Whilst looking around the Fairy Tail tag, I was looking again at a question here asked by Shinobu, I finally decided to pop the question "How old is Natsu". I thought it would have been revealed further on in the series, but from the looks of it they haven't although the ages of Lucy, Erza and Gray have been revealed which is as follows
Has an age ever been given in the mangas to how old Natsu is?
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.
I've never seen it mentioned, specifically, but we can get close to a minimum age.
July 7, X777 is when the dragons disappeared. Since Natsu was trained by Igneel, he must have been born before this date, presumably several years before to allow time for training and such. If Natsu had been born the day before, he would be about 14.
During the Fighting Festival arc, the Battle of Fairy Tail specifically, Natsu, Gajeel, and Makarov are trapped by Freed's enchantment, suggesting that Natsu must be older than 80 at this point.
The Edolas arc sees the return of Gildarts after 3 year absence, suggesting the Battle of Fairy Tail was less than 3 years prior to this. Since there is no minimum time that passed, there is not much help to us here.
In the X971 arc we know 7 years have passed, so Natsu must be at least 87 by this point.
Beyond that, I'm not sure how much time is passing between each event.
Speculation
I do have a theory that would make sense of why Natsu is so old when physically he looks young:
Natsu and the other Dragon Slayers are actually their respective dragons. For example, Natsu is actually Igneel.
From this it can be concluded that Natsu is at least 14 years old. That is the only thing we can know for sure. But as to how old he really is, there is no definite answer. The Battle of Fairy Tail suggest that he might be older than 80 years old, but it was never mentioned explicitly that he is.
It is safe to assume that something inseparable from Natsu (his magic, a sealed dragon, previous sealling like the 7 years thing) could have also kept him from passing through the enchantment. Ergo, the 80 years seal cannot nessesarily be used to develop a lower bound for his age. In addition, he doesn't truely age when sealed away for 7 years. (Lucy was far too... developed... to be 10 when she joined Fairy Tail).
For a lower bound of his body's age, we have to note that he remembers Ingeel. Most first memories a child has are from 5 to 10 years of age (i'm the latter). Their first clear memories are definitly around 10 years old. Assuming Natsu must have been at least that old when Igneel disappeared, means that in year X784 (when the series starts) Natsu would be roughly 17 years old by this specific means of estimating his age. This puts him around the same age as the others which makes sense. If he lived much longer than that, (living time) he would remember having lived much longer than that and would likely look like he had lived much longer than that.
I don't, however, believe his timeline is that simple. Zeref seem to remember him which means he should be several hundred years old. The dragon slayers all seem to have indeterminate ages that may be easily over 80 years old (as shown by the previously mentioned enchantment).
So define living time as time he is conscious and sealed time as time he did not age. Total age is living age+sealed age.
We do not know his age (living or sealled) as it is not yet revealled. His living age is roughly 17-19 years old close to other main characters. This seems to be the measure used to define Lucy and the other's ages in the question. His total age is at least 24 as he was sealled for at least 7 years. His total age, however, as no upper bound and there is evidence that he (or part of him) could be several hundred years old. The discrepency between living and total age, however, all seems to be time sealled away by some yet undefined means.
I would assume that Natsu is around the age of 17-18, because when he was fighting Gray and when he joined Fairy Tail, he looked about the same age, as I don't think that all the time travel with Eclipse would have affected it, however, if they had the 7 year gap, so technically Lucy would be about 24, but I think that we can say he is around the same age.
Also, it wouldn't make sense that he is 70 something, because, yes, the age of dragons was around 400 years ago, but when the training is shown, he is the only one there, not to mention the fact that Dragon slaying is a lost magic, and is quite rare. Before the phantom lord incident, he didn't know that other dragon slayers existed, and as a boy he didn't know how to read and write, so he was quite clueless, surely in 70 years, he would have learn things!
In conclusion, if Guildarts left when he was 70/80 in a child form, and returned 3 years later, he was in the state we see him now, considering that Erza and the others grew at the same rate.
Natsu couldn't be 14 because he didn't meet Igneel when he was born. When Igneel found Natsu alone in the woods, Natsu was able to walk properly, which means that he had to be around 7. Igneel left when Natsu was around 12 and spent around 5/6 years in Fairy Tail, not counting the years he skipped in the Eclipse. Hence, Natsu is currently around 17 or 18.
Gildarts returned from "Century quest". That means he was gone for 100 years, but he was with Natsu when he was little... so I think that in their time 100 years are maybe 3-5 years in real life. So... They are maybe several hundred years old but they just don't age as quickly as in real life...
It would make sense - the age of dragons was 400 years ago so if 100 years = ~4 years then Natsu is around 16 + time before he met Igneel so around 17-20 years old in our world. Or like 500 years old in Fairy Tail universe!
Or... maybe they just count years in different way. Maybe "one year" is for example 50 days long. So 365 days are 7 years not 1 or something... that's why they put for example X780 year. That X is a number that changes not in 1000 years but faster.
Hope you liked my theory :)
All the current Dragon Slayers who were raised by Dragons have there parent Dragon inside them. That's why it seems like they are more than 80 years old.
Natsu is older than Wendy, but younger than Erza, so his age would range from 14-17. I'm leaning more towards the older side.
Well it is hard determine the age as the dragon slayers have come from 400 years in the past.Technically Natsu is about 417-419 years old
