How old was Natsu when he was sent forward in time? I know Natsu was 7 (or so speculated) when the dragons left. I know he was sent forward in time when that happened. However why doesn't Natsu remember what the world was like 400 years ago? I mean I think he would notice a difference... even if he was only 7. Was his memory altered or something?
I don't think there was much of a difference between the world where Natsu went to sleep, and where he woke up.

For the most part, he spent his time with Igneel in the woods - as I assume a dragon wouldn't be welcome in towns.

I think Natsu grew up in the wilderness, went to sleep in the wilderness, and awoke in the wilderness 400 years later, not noticing a change because the wilderness couldn't have been different.