The problem of youth has one other sort-of-tangential problem for me: class.
My first thought is to make the Edward figure (Daniel, from here on out) to be the son and heir of a CEO of a big company who dies early on in the play, or even before it begins. But that gets rid of lots -- lots -- of the weighty material of Marlowe's work. Actually, the more I think of it this way, the less I like it. I think Daniel will have to be a young king. I mean, there /is/dramatic precedent for that (cough cough Henry IV 1, Henry IV 2, Henry V cough cough). And it does give a certain frisson in the relationship between Daniel and some of his nobles: in Marlowe's version, I think there's a certain petulance and conception of Edward's inexperience by the nobles that's difficult to respect in a man the age of the historical Edward II, but completely in keeping with someone 18 or so.
After all, it has been argued that this kind of generational conflict is a big theme in the work (I think the editors of the New Mermaids version [the 2nd edition] of the play bring that up), and I kind of like the idea of the older generation of rebels (Mortimer Senior, Warwick et al.) being infirm in some way: morbidly obese, with an oxygen tank, in a wheelchair... Remember, the characters aren't on an even playing field, so of course these lot get the short end of the stick.
Which leaves the problem of the Gaveston-character's age (Michael, from here on out). Gaveston is a well-written character in Marlowe, so there are several ways he can be played by an intelligent actor. Very often, he gets played as an opportunistic schemer who takes advantage of Edward (Derek Jarman's Edward II film shows this to a degree); this usually shows Gaveston as older and wiser than Edward. There are also versions (like Brecht's play) that show him as the object of Edward's unreasoning passion, where he often is shown to be younger than Edward.
I think, though, there are problems with both interpretations. Each version gets played by reducing the agency of the opposite lover: the more manipulative Gaveston, the less canny Edward and vice versa. I think the best path is to go in between. There's a great scene in Brecht's version where Gaveston runs away from a battle that really doesn't work in his vision of the relationship. It's a scene that's touching on its own, but doesn't ring true for that mise-en-scene. (My thought is that it's some unadulterated Lion Feuchtwanger, but I've got nothing at all to back that up.)
All in all, I think Michael should be a little older than Daniel; the real Gaveston met Edward when Edward I was impressed with Gaveston's character and recommended him to Edward II as a companion. Being a little older keeps that idea going, but also helps play up the historical idea that Edward I later repented himself when Gaveston turned out to be a bad influence. And I think bad influences are sexy.