〜ともなると／〜ともなれば

Marks a special stage, age, position or season and states what naturally follows at that level — usually something different from ordinary circumstances. The speaker treats the situation as high-level or exceptional, so it does not fit trivial, everyday conditions. Common in writing and commentary; 〜ともなれば is slightly more conditional in feel but the two are interchangeable. Compare 〜となると, which is broader and can describe any 'if it becomes the case' situation without the sense of a raised level.

• 連休ともなると、この道はひどく込む。

  れんきゅうともなると、このみちはひどくこむ。

  Once it's a long weekend, this road gets terribly crowded.

• 大学生ともなれば、自分のことは自分で決めるべきだ。

  だいがくせいともなれば、じぶんのことはじぶんできめるべきだ。

  Once you're a university student, you should decide things for yourself.

• 部長ともなると、責任の重さが全く違う。

  ぶちょうともなると、せきにんのおもさがまったくちがう。

  When you become a department head, the weight of responsibility is on a whole different level.