Well, he'll be 49 by the time the next record comes out... maybe even 50. Megadeth is an intense Thrash band (duh) and it's taking its toll on Dave and probably has for a number of years now. I read in an interview that he said his hands are "disintegrating" because of playing guitar. He probably means his fretting hand can't handle the movements anymore.
It's cool Dave has continued this far, considering he was contemplating ending Megadeth after releasing The System Has Failed. He announced to the crowd at a show in Argentina (That One Night DVD, AAMOF) that he would continue Megadeth.
But it seems like Dave wants to rush this last record. I remember reading Dave was talking about the next record only a few months after Endgame had been released.