He started learning to play the piano in his 40s.

40s 是 forty 的复数形式，表示"40多岁"这个年龄段。英语中常用"in one's + 数字复数"的形式来表示某个年龄段，比如 in his 20s（20多岁）、in her 30s（30多岁）等。