Are you asking if "youngster" and a "teenager" have the same meaning? If so, the answer is "no." A "youngster" is a child or young person. A teenager would be someone in the range of 13 to 19 years of age.

Youngster is a relative term. You can be a youngster at 50 years old, relative to a 90-year-old. Teenager is someone in a specific range of years.

hmm.. younger is more than 20 y while teenager is under 20y.. that's from me[emoji]

No. Younger can be quite a broad term. For example, you could say/write: "A sixty year old is younger than a ninety year old." "Teenager" only applies to those between the ages of 13 and 19.

"Teenagers" are people who are typically between 13-19 but "younger" has a broader meaning, that simply means someone who are literally younger than someone else. My father (50) is younger than my grandfather (80), and it means that my father is not as old as my grandfather, although my father is not young anymore. So, a teenager is always a young person but a younger person is not always a teenager. :)