Meaning?

Hi , what is the meaning of he made partner at thirty-one in the following text?

They (hausband and wife) quarreled constantly, so he put in even more hours at the office.He( the hausband who is a lawyer ) made partner at thirty -one and was knocking down close to a million a a year.

Does it mean that he became the partner of the firm at the age of 31or he got 31 percent of the shares of the law firm which he was working with?

Thanks