My parents had already travelled the world by the time they were in their forties.

in 在这里表示"处于某个年龄段或时期"。 their 是形容词性物主代词，指代"他们的"。 forties 是 forty（四十）的复数形式，表示"四十多岁"的年龄段。 in their forties 这个短语表示"在他们四十多岁时"，指从40岁到49岁之间的年龄段。