In my travels in the USA (on Long Island), I noted that the pronunciation used by a teenager sounds different from the pronunciation used by an adult.
Does such difference exist, or is it just my impression?
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This is actually common across languages. Younger generations speak differently from older generations, and not only in vocabulary and syntax, but also in pronunciation.
The answer to this will massively depend upon where in the United States you travel. Most dialects in the US are undergoing changes in progress, and are they not all changing in the same way (again, see the Atlas of North American English).
When a language changes, by definition that means that the children in a community speak differently from the adults. However, how they speak differently will depend on the direction of the change. So, in the American West, there is a particular change that has making short-a sound backer, so that "cat" is sounding more and more like "cot". In the Great Lakes region, the exact opposite is happening, where "cot" is sounding more and more like "cat".
In both the West and the Great Lakes Region, children sound different from adults, but in completely opposite ways.