I think this one differs between British and American English. I'm American, and I always stress the second syllable (uh-DULT). EDIT: You can play the sound clips on the Cambridge dictionary website to hear the difference between the UK and US pronunciations - https://dictionary.cambridge.org/dictionary/english/adult

There is no fixed convention for the stress pattern in the word 'adult'. Some people always put the stress on the second syllable, some always put it on the second, while others make a distinction between the noun and the adjective. Personally, I always stress the first syllable.

both are correct

From what I hear in daily life in the UK, around 30 years ago it was quite common to hear the stress on the second syllable. However it seems to have almost completely fallen out of use, and nearly everyone puts the stress on the first syllable of 'adult' no matter how they are using the word