I made an older thread with the theory that Termina was created for Link (and maybe, to heal Skullkid, destroy Majora, etc.) so that everything that happened in
MM was intended to by the Goddesses, who created that world instantly. The people there who have memories, are real and living, but their memories are fake, whereas they only just started living when Link entered Termina, or maybe shortly before, or maybe even when Link was born.
HERE is that thread with the details, (don't post in it, it's far more than a month old).
EDIT: This explains the Romani/Cremia inconsistancy, they are two characters, but share the same image for the same girl from
OoT. The only one in the CT who would have seen an adult Malon at the time would be Link, who travelled there and back.
EDIT 2: Wow, I forgot to add my insight on the purpose of this thread
I'm unsure of what I believe FD to be, but I really like the idea of the being and mask being a reflection of Link's soul at the end of the game. In the beginning, he is transformed into a weak Deku, and the end where he gets an almighty mask it would signify his growth and power. He was able to mature naturally throughout the game, and justly contain the adult spirit inside his body. He grew and became an adult in his own right. The FD Mask signifies this.