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 sence- 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 year old in Fairy Tail universe!
Or... maybe they just count years in different way. Maybe "one year" is fir example 50 day long. So 365 days are 7 years not 1 or something... that's why they put for example X780 year. That X is number that changes not in 1000 years but faster.
Hope you liked my theory :)