15 is pretty young for an Avenger, isn't it?
I figured Valeria would be younger than Franklin, but only by a couple of years. I think it would be most fun if they were close in age.
However! Even in canon, Valeria's appearances are muddled by time travel and temporal effects. She could be from the future, or from a parallel timeline...so there's room for fudging.
We should also check with the GM and other PCs at this point. Advancing the age of the Richards kids makes a lot of Marvel heroes older as well.
And both Franklin and Val had exceptionally fast mental development, so it might not be as much an issue as wanting to play, say, Jubilee. Still, it would lock out certain plots that might otherwise be entertaining.
I think, so long as no one's wanting to play Doom or the Fantastic Four themselves, you're probably safe with whatever ages you choose. Since the game already involves rewriting history, the only characters whose ages would be tied to yours would be your own parents; everyone else can simply have a different origin time as part of their backstory (or the FF / Doom could slide back in time for theirs, probably easier) to account for why 'your' Richards kids aren't the same relative ages as other contemporary Marvel characters.

Are we playing in the past, present or future? I've written Gambit background using some actual items of the actuality and I have assumed that the game would be either present or near future.
I thought interesting to put some Super-Villain source to all those disaster that have strike New Orleans.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.