Harden was born in 2267. He is Lucas Simms's 10-year-old son. His father has instructed him not to talk to strangers in Megaton, and he reluctantly obeys. He goes to sleep early and rises early, which gives him enough time to meet up with Maggie and play.
His father died at an unknown point in time.
By 2297, Harden was the sheriff of Megaton. When drunk, he would often speak about his father's death, and blame it on the Lone Wanderer. 
He's a good friend of Maggie and is usually seen near her in the mid-morning to late afternoon. It is implied by his father, Lucas Simms that Harden also polishes Deputy Steel on a regular basis, but this is idle chatter as the child is not scripted to enter the Megaton armory.
The Power of the Atom: If Lucas Simms is dead when you disarm the bomb, Harden will reward you with a home in Megaton and any caps you requested at the beginning of the quest. He exhibits little emotion despite his father's death. If the bomb is detonated, he and Maggie supposedly die, making him one of the only 2 killable children in the game.
↑Afterward - Moira Brown: "Just shipped out another crate of survival guides, and the caravans just can't seem to get enough of them. Sheriff Simms says they've really put Megaton on the map—pretty ironic, since I've had to redraw that map about a million times since the first edition came out 20 years ago."