User blog comment:Stone Roger/Natsu and Zeref have somekind of relation?/@comment-101.161.34.91-20140201135739

Hmm, I wonder if maybe Natsu has previously been frozen (like on Tenrou Island) as a child and during that time Zeref had changed so that Natsu couldn't recognize him. It would also make sense if you think back to the fighting festival arc when Natsu couldn't get past the enchantment that didn't allow people over 80 to proceed. He would be biologically be young, but old chronologically. It may have happened to all dragon slayers besides Natsu too.