Are the people putting stress on the second syllable native speakers? Native speakers put the stress on the third syllable, at least I've never heard a native speaker who didn't. This applies to British English, and I think to American English too.

The stress is on the third syllable (..LESC.. ) If you have heard anybody stress the second syllable they are wrong. The second syllable is not pronounced as a 'do' sound. It is an unstressed schwa vowel, written like / ə / in dictionaries, and sounding like a very short, weak 'duh' sound. It is never stressed by native speakers.

I've never heard the accent go anywhere but on the third syllable. But this is exactly where a dictionary is useful. For U.S. English I really like the American Heritage Dictionary--I have the print volume but the online website seems to be about the same. English dictionaries will list all of the usual pronunciations, the most frequent first. https://ahdictionary.com/word/search.html?q=adolescent&submit.x=45&submit.y=17 In this case, only one pronunciation is shown and the accent is shown as being on the third syllable: ad·o·les·cent (ăd′l-ĕs´ ənt) For contrast, let me check the word "kilometer," which I pronounced with the accent on the second syllable, but a friend of mine pronounces with the accent on the first syllable. kil·o·me·ter (kĭ-lŏm´ ĭ-tər, kĭl´ə-mē′tər) Sure enough, mine is the first listed, his is the second. Lexicographers are good at their job.

It is the ES in the meddle. Adolescent [ad-l-es-uh nt] check dictionary.com