It is well documented that polish children take longer to become efficient in their own language somewhat due to the amount of declinations and volume of vocabulary. they even have 'child speak'.
It has also been proven that adults are able to pick up grammar and vocabulary at a much quicker pace due to memorisation and linkage to previously learnt vocabulary.
So here it is: Polish grammar would take less time to master as a young adult than a native child irrispective of fluency in comparison to a language such as French, English etc etc.
what do you guys think?
That is a very interesting topic. Do you have a link to the studies you are referencing? I would love to read more.
Several questions I would raise: Is the "child speak" you reference really that different from how young children speak in any language? The English-speaking children I know also speak differently from adults: more simple language, and with certain common child's grammar mistakes — children continue to learn/improve their language skills through all the school years, but even as children, they are completely fluent in the sense of being able to speak effortlessly and be understood. I have always presumed the same is true for children of any language. Is it really so different for Polish children?
Secondly, while it may be easier for an adult to memorize declination/conjugation charts and other grammar rules, how easy is it for an adult to internalize those rules to the point of fluency? I.e., being able to simply produce sentences at a conversational rate of speed, without needing to think back and reference the rules they have memorized?
And lastly, I think you do have to take into account the learner's native language. For instance, it would presumably be much easier for a Czech native speaker to learn Polish, than a Mandarin native speaker, because of similarities in the vocabulary, grammar, etc. the person already knows.
I am not trying to challenge you. You have definitely piqued my interest with your topic. I just think there are still a lot of questions to be answered before reaching a conclusion.