What is Youngster?

The Youngster achievement is actually awarded for catching some number of Rattata below a certain weight.

The achievement previously stated "Caught {0} Rattata weighing at most {1}". In many programming languages, {0} and {1} are just ways to substitute numbers into some text. It seems that Niantic simply messed up the description, even though it tracks correctly.

A screenshot may be found at Timelord64's answer to the question I've got an “XS” Pokémon. Is it special?, reproduced here:

The badge is called BADGE_SMALL_RATTATA in the data dump. The data dump lists its targets as \0032\254\002, which translates to requirements of 3, 50, and 300. There's no indication for weight, but I suspect it just requires XS Rattata.

Regarding XS Rattata specifically, I hypothesise that a Rattata is XS if it is 2 standard deviations below the mean. The mean for Rattata is 3.5 kg, and the standard deviation is 0.4375 kg, so you should be getting it for Rattata below 2.625 kg. (The game only shows two digits for weight, so this falls between 2.62 and 2.63 kg. I hypothesise this because I have a 2.4 kg XS Rattata, and a 2.75 kg regular Rattata.)

Items {
  TemplateId: "BADGE_SMALL_RATTATA"
  Badge {
    BadgeType: BADGE_SMALL_RATTATA
    BadgeRanks: 4
    Targets: "\0032\254\002"
  }
}

Although it doesn't say so in the description anymore the badge is supposed to be awarded for catching lightweight Rattata. The name BADGE_SMALL_RATTATA is pretty obvious. It's the opposite of the Magikarp badge, where you need big catches to get it.

Some communities confirm the text to be bugged, some say it's the badge itself, that's bugged. My recent Rattata catches showed a working badge with incorrect tool tip text.