https://imgur.com/1NkRZq4
Isn't Duolingo an app for every age?
https://play.google.com/store/apps/details?id=com.duolingo
I know they aren't in charge of the advertisement and they do it via a third party, but what the hell... What if they show this to a minor? What if a child was learning a language on my phone? :)
It's similar to Youtube in that regard. An "Everyone" rating on play store doesn't mean an app meets regulations related to user age, you need to read the EULA to understand where an app stands legally. For example: letting the user upload a photo avatar runs afoul of COPPA.
As for why you're seeing that ad, it has to do with browsing history tracking by ad networks... (I started to get contextual ads after looking at a specific niche elsewhere myself)
(Disclaimer: my brother works at Duolingo)
They are just advertising it that way, and I think it makes it even worse.