Considering that the historical setting of the play takes place in the late 1940's (circa 1949 when the play was actually published) we could assume that, twelve years later, would make it 1961-1962, and Biff would be 46 years of age. Happy would be somewhere around 44 years old.
Twelve years prior we find Biff finally living through the epiphany of understanding that his life, up until his 34th year of age, had been a lie caused by Willy Loman's dreams. As a result, Biff takes the initiative to end with the lies of his life and even break from the curse of lies and deception that Willy Loman had brought into the family while he was alive.
Will you let me go, for Christ's sake? Will you take that phoney dream and burn it before something happens?
It is safe to assume that, since Biff chooses at Willy's funeral to break away from his father's legacy, he may have very well moved away to go find himself. This time it would have been easier to move since now he has the funds that he lacks before. He surely embarked in that dream he has of working in something outdoors where he feels free to act the way he wishes to.
At first, it would surely be a hard thing to do to move away from Willy's frame of thought. However, once Biff realizes how well he can do on his own, he will find his way.
Happy, however, never experiences any epiphany and remains a flat character throughout the play.
Biff: Why don't you come with me, Happy?
Happy: I am not licked that easily. I am going to stay in this city and I'm gonna beat this racket....The Loman Brothers! [....] Alright, boy. I am going to show you and everybody else that Willy Loman did not die in vain. He had a good dream. It's the only dream you can have! To come out number one man. He fought it out here and here is where I am going to win it for him!
This means that Happy, as well as Linda, are meant to perpetuate the same silliness that made them stagnant in the first place. Linda surely would want to see Happy trying again to fulfill Willy's dreams, although they were the wrong dreams. However, neither Linda nor Happy ever change. They will likely go after the empty dreams of Willy Loman and lose themselves in them one more time.