I have studied music since I was twelve or, in other words, I started to learn how to read music and how to play an instrument about ten years ago.

or 在这里是连词，表示"或者说"，用于引出另一种表达方式或更详细的解释。 in other words 是固定短语，字面意思是"用其他话说"，用来表示换一种方式表达相同的意思，通常用于进一步解释或澄清前面的话。