Answer:
Sensitive
Explanation:
According to my research on studies conducted by various developmentalists, I can say that based on the information provided within the question Sergei's story illustrates a term referred to as a Sensitive Period. Based on developmentalist's research this is defined as a time when the effects of experience are particularly strong on a limited period in development
I hope this answered your question. If you have any more questions feel free to ask away at Brainly.