Difference between 那时，那时候，当时，那会儿，那阵子
那时/那时候 - formal, a long period of time. Whichever one you choose depends on how fast-paced you want your narration. 那时，我开始对数学感兴趣 makes the most sense because your growing interest in math sounds like a long-term process.
当时 - formal, a very short period of time. Example: 当时，我吓坏了。
那会儿 - colloquial, any length of time. Example: 那会儿，我在火车上只吃方便面。
那阵子 - colloquial, a very long period of time. Example: 那阵子，我特别无聊。