If the Monsters...

If the monsters are the people who sacrificed their children in exchange for immortality, then why do they all look like they come from the 1950's? If this seems to have started sometime around 1506 (the earliest date we see in the lighthouse). I can buy the early settlers theory as we've seen paintings of pilgrims and visions of people in period clothing. One of the Monsters is an old woman, Smiley is a young man, the one who asks Julie if she remembers him is a teenager who looked to be in modern clothing. Would they all have had children to sacrifice? Or are new monsters created somehow?