You're right, Havok would be around 40. Maybe 1-2 years younger or older.
So his range is say 38 to 42.
If the focus of this movie is a teenage to early 20's Scott, Jean and Ororo; then no thank you on a 40 year old Havok.
And yes, if Gambit makes it in he'd have to be somewhere around mid-20's, right? I wouldn't mind that IF it played into the story. For example, just like in the comics, when an older Gambit and a young (at the time), teenaged Storm, met and became friends. This could work.
But just having a mid-20's Gambit in with mid to late teenaged Cyclops, Marvel Girl and Storm; would just not make sense...
Now, with all of these people indicating they want a 20 year old Jean, Ororo and Scott; let me ask you this... Don't you want to see Charles go out and find these three (and maybe Nightcrawler as well) and bring them back to the school to help them develop their powers? Wouldn't it be great to see them in a Beast invented training room?
If yes, then wouldn't mid to late teens make so much more sense since that's when their powers usually develop. Waiting until they are 20 something just makes no sense. Yes, they could have them brought to the school in the early to mid (maybe late) teens and then jump a few years to the future in order to make them 20 something, but that kills the whole school feel, which is such a great thing for the X-Men universe.
No, please give me a mid teen (18 latest) group of Scott, Jean and Ororo, and have Nightcrawler in there as well, since his age will pretty much work (he'd be like 19-20). No thankyou to 40 year old Havok, unless he's a Horseman, but yes please to a mid-20's (older brother relationship) Gambit, as long as his older brother relationship is with Storm.