Perfect, now we have a common definition.
So let's see:
> Poland studied this, and found about 40% had lied about their age in the refugee wave 2007-2018.
You might not be able to read Polish, so I'll help you understand this article.
It refers to a German investigation (your link is just a Polish news outlet citing the German investigation) done by the University Hospital of Münster.
They found that 234 cases [1] (that's the 40%) in 12 years, of all cases that were reevaluated had lied about their age in order to get better chances at asylum approval. However, they only included cases for re-evaluation that were already looking suspicious.
It's not "40% of migrants lie about their age". The "study" was not published or at least isn't online.
So let's see what happened.
The Merkel administration had to deal with a surge of asylum seekers.
Some of them claimed false ages to get a better chance in the approval process.
Some courts and Jugendämter in Germany noticed that. People at UKM in Münster re-analysed some suspicious looking cases, and talked about methods for improvement of age determination.
Wouldn't you agree that the system worked? A problem was diagnosed, and measures were introduced to solve the problem.
Now, the AfD's trick is to not tell their voters what's happened since then: The numbers for people seeking asylum went downhill after 2016, so the problem didn't exist anymore, at all [2].
What we're dealing with now are mostly war refugees from Ukraine.
Now, AfD's trick is now to convince people that the amount of fake refugees in itself is problematic, without giving specific numbers, and without telling how it's problematic, and why. They make it seem as if it's a real problem that exists, and as if it's being ignored (it's not, as we see by the scientists trying to solve it)
[1] https://www.zeit.de/gesellschaft/2019-09/studie-minderjaehri...
[2] https://de.statista.com/statistik/daten/studie/76095/umfrage...