Condition and supposition

～ともなると / ～ともなれば

once it comes to; when it gets to the level of

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How to form it

Says that when a situation reaches a certain stage or status, a natural consequence follows.

The noun is a higher status, age or time: 社長, 大人, 年末. The consequence is felt to be natural.

十二月ともなると、街はクリスマスの飾りでいっぱいだ。
じゅうにがつともなると、まちはクリスマスのかざりでいっぱいだ。
By December, the town is full of Christmas decorations.
社長ともなれば、責任も重い。
しゃちょうともなれば、せきにんもおもい。
Once you are president, the responsibility is heavy too.
子どもが三人ともなると、食費も大変だ。
こどもがさんにんともなると、しょくひもたいへんだ。
With three children, food costs are a real burden.

～とすれば / ～とすると: とすれば is a plain supposition; ともなると adds the idea that a higher stage naturally brings this result.