You realize you are basically agreeing with me right? That it is a number, in the teens, with no reference to maturity. That once you are 18+, regardless of the number of your age ending in 'teen', you are an adult. and 3 can be considered a couple btw;
4: an indefinite small number : (https://www.merriam-webster.com/dictionary/couple)
while most of the time it is reference of 2 of something, it is not strictly limited to it either.
also, there is a think called a bakers dozen, it is 13.