A teenager is a character between 13 and 19 years old. This is the age of mid-adolescence and mid-pubescence.
Puberty can begin (but generally does not complete) prior to being a teenager. By the time one ceases to be a teenager (age 20) puberty has generally completed.
This wiki's focus (and requirement) is "young teens". The top two years (18 and 19) are the "late teens" aka "old teens" and are generally considered legal adults (age of majority), so there they are not subject to misinterpretation of anti-CP laws which lead various sites to forbid their use.
The terms "early teens" or "young teens" of 13-17 are the ages that Dolores Haze (Dolly aka Lolita the beloved Nymphet of Humbert Humbert: the etymological origin of the terms "Loli" and "Lolicon") was in throughout eponymous tale by Vladimir Nabokov.
Lolita was pregnant at 17 years old and died during childbirth on Christmas 1952 as Mrs. Richard F. Schiller. This is why the upper crux of "loli" is considered 17 years old, which coincides with where the Age of Majority is in most 20th century countries in terms of determining pornographic legality.
Although the core/original meaning of "loli" referred to a mid-pubescent/mid-adolescent girl, broadened usage of the slang in Japan to refer to younger (pre-teen pre-adolescent) characters has introduced an emphasis to label "teenage" to narrow it down to teen lolis 13-17.
The next 5-year span below this one would be 8-12, and below that 3-7. The span of 1-2 is considered the "toddlerkon" form of loli/shota.
Tween is a subset of preteens who are "between" some unestablished earlier point and teens. This might be 8-12, 9-12, or 10-12 years.
Due to age progression and age regression in art, this tag is often used even when it does not line up with canonical character ages, based upon the interpretation of characters as to how a character's body appears.
This could also apply to characters who always appear with a teenage body type even though they're canonically much older.
Conversely, sometimes actual teens (who deserve the tag) get drawn with embellished boobs which may lead people to think they are older, but actually you just add oppai loli to indicate that inflation if the rest of their body is otherwise neotenic.