Quote Originally Posted by Iron Maiden View Post
I think what throws a monkey wrench in things is the birth of Franklin and Valeria. Now they both are getting to be as tall as their parents. So Reed and Sue became engaged in Fantastic Four #35, got married annual #3 then Franklin is born in annual #6. Marvel kept him at the age of 4 or 5 for a long time to keep everyone from aging to quickly. Valeria, the one that is now in existence, was born in Fantastic Four #54 volume 3. We won't even get into Sue's miscarriage in Byrne's run and how Valeria Von Doom fits in.

These days it does seem like 15 years has to be the sliding time range but soon it will have to increase. The kids seem to be getting older at a faster rate or maybe Marvel just wants to make that one jump and leave them at their current age, whatever that may be, for now.
15 years appears to be the common period of the Marvel Age of heroes. That is, the FF getting their powers. Regarding Franklin and Valeria Richards, they both spent years away from the 616 Earth and aged at different rates. As far as an accurate metric, they're not really very good. But then again, there really is no official in canon length of time and we seem to be best just going with 15, at least for a while. I do recall 13 was used in the dame unofficial way a few years ago.