It's like playing a musical instrument (or learning a martial art or style of skiing or a number of other skills): every time you know one more well,* the next one comes more easily. It is generally thought that children enjoy a full access to Universal Grammar which adults do not, but the evidence is not unequivocal in that regard.
*The caveat is well. Going on to the next one before getting a good grasp on the one you have started tends to lead to confusion.
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