My opinion is that since language acquisition (whether native or others) is generally known to be easier for everyone during childhood, this shows that there are biological/genetic factors (in this case would be that of a person's development stage) which contribute to a person's language aptitude. Nonetheless, my belief is that the ability to learn other languages is not genetic as similar to aptitude in other areas, nurture is a more contributing factor compared to nature. For instance, a mother who does not have a high language aptitude may predispose a child to a similar level of language aptitude but it does not necessarily mean that the child will be inapt in learning languages. However, there is a higher tendency of a child being unable to learn a new language effectively due to the fact that the exposure level to that language is low. For instance, I am currently tutoring a primary school student Malay language. Even though her mother is proficient in Malay, due to the fact that her father is not Malay, most of the time at home she does not practice or even hear that language and hence her literacy in Malay is rather poor.
PS: My proficiency in Malay Language is also declining due to lack of usage
Disclaimer: This is my personal opinion and I might be wrong