I believe 在 means -ing here... It is used to emphasize it is 'progressive tense' 我在吃饭呢！(I am eating now! ) 他在睡觉 he is sleeping. 你在说什么！！ (excuse me?)What are you talking about?

Yes, the sentence means: ---- I didn't understand what adults were talking about when I was younger. Just like “你在做什么？what are you doing?” “在”here is used to stress a moment (when the adults were talking). It's totally the same to English. Literally it only tells us that he was not able to understand adult's word when young. But usually, now, since he's grown up, we tend to take it that he is able. It 's nothing to do with "...only ... but now NO".

在can be what you are doing/where are you 她在睡覺 She is sleeping 我在東京 I'm in Tokyo

Feel free to contact me

Hello, This is William from China. 在 here indicates continuous tense，which means doing For example, 我在吃饭， this means I am eating. 他在演讲， means he is speaking "我小时候" indicates this case happened before, not now. Now I already understand what adults are talking about. So your understanding is totally right.