ともなると / にもなると

ともなると follows a noun for a high rank, a special stage or a large amount, and says that once things reach that level, what follows is only to be expected: "once you're", "when it comes to". にもなると means the same, and both have a ば version, ともなれば and にもなれば.

The noun has to mark a step up from the ordinary: a role, an age, a time of year, a long stretch of time. The second half states the consequence as a fact or a judgment. It is not a request or a decision of yours.

• × 社長ともなると、頑張ってください → 社長になったら、頑張ってください

ともなると or となると

となると takes any noun or clause as a premise: "if it comes to that". ともなると adds that the noun is a level above the ordinary, so the result follows as a matter of course.

ともなると is formal and mostly written. In conversation, になると or くらいになると says the same: しゃちょう社長くらいになると、いそがしい忙しいよね.