Old!Fai would be really happy to find a version of him that hasn't been dragged through every horror the sadistic minds of certain mangaka can come up with more or less hell, and who because of this is a rather honest and forthright person. On the other hand, he would be absolutely horrified to learn that Fay doesn't have a twin, and that there are no signs of that he exists in his world. Fay also would literally make him feel pretty damn old, since he's had a normal human lifespan and not lived for hundreds of years.

Oh, and he would probably slap Fay across the face because he skipped out on his family and stays away out of fear. Because seriously he actually has a family from the start that doesn't want him dead, and should fight for them.

New!Fai/Fay would be confused and a little bit unnerved to meet a version of himself that has been so considerably fucked up by such an amount of suffering, and find the pretendy smiles and lingering tendencies towards lying pretty damn annoying. He would also be kind of deeply embarrassed by Old!Fai's huge lack of ihibitions in almost every aspect save the emotional, and weirded out by his way of interacting with... well, everyone. Basically he would interpret the whole thing as seeing a version of himself that has been broken by life and therefore gone kind of crazy, and would not find it to be a nurturing experience.

If he remembers Infinity at that time and the way old!him treated Kurogane, he would definitely return that slap. Because seriously what the fuck.

...he would however be vainly pleased that the Echoes will mean that he probably won't age much. Silly man.