system
Given the following post, respond as if you're commenting on Reddit:
human
[Fallout 3] Why does the Lone Wanderer already start out as a certified badass?

It’s only been an in-game week and my LW has slaughtered hundreds of raiders, feral ghouls, super mutants, and angry krabby patties all throughout DC despite it being his first time out of a vault as a 19-year-old who has presumably not had any other experience with guns besides a BB. Sure there were plenty of vault dwellers  who made excursions into the wastes from time to time, but obviously it was different with Vault 101. There would have been no purpose for James or the Overseer to have trained the LW to face anything deadly outside beforehand either. Like shouldn’t he be crying and pissing himself at the first sight of a super mutant?
gpt
It's established that Roaches occasionally made their way into the Vault and the LW killed them as a kid with the BB gun, we can assume they'd do it from time to time, even if for entertainment (Vaults must be boring as shit), even if with a BB gun, they're training and improving their shooting skills, during their way out of the Vault and likely every now and then, they saw Vault Security using their 10mm Pistols, so that adds up.

We saw that during the breakout, roaches began attacking civilians and at least 2 of them, Butch and his mom, weren't prepared at all to handle it.

So we have a young adult with decent shooting skills, somewhat familiar with the Vault weaponry and a Pipboy that helps him aim, so he makes his way through the relatively safe path from the Vault to the nearest Town, where he'd inevitably see more guns, either with other scavengers or for sale, being allowed or taught how to operate them would add up.

With that, they're already better off than the regular raider, when they first see Super Mutants, they have a BoS Squad there to back him up.

From there onwards, it's a matter of the LW gathering better gear, making the right allies and he's a badass.