Yes, I too see the shortcomings in believing that Sorceresses age differently. Basically, the main reason people seem to believe this is because Edea looks much younger than cid. Let me point a few things out:

1. In many Japanese games/anime, the female characters appear youthful and pretty, despite their age (Bulma in Dragon ball Z is proof enough of this). Male characters are not usually given this luxury, thus it possible for 2 people approximately the same age to look years apart.
2. As can be seen from the similarities in the character designs of the female FF8 characters in the FMV’s, the same basic body seems to have been used for all of them, with minor alterations. They probably couldn’t be bothered changing her appearance enough to make her look any older than the other characters.
3. In Japan, it is seen as more acceptable for an older man to marry a younger woman, so it’s entirely possible that the age difference was always there, and not caused by slow aging.
4. Cid looks like Robin Williams. Not sure what that proves, just thought I would through it in there…

As you can see, there are major flaws in believing that Edea has aged differently to anyone else, thus there is no basis to the idea that Sorceresses live forever.

One last thing though…

Laguna: … Ultimecia lives far in the future where none of us
can technically exist.
Direct quote from the game. Don’t you think it’s a little odd do believe in a theory that is DIRECTLY refuted by the game itself? Just a thought…