〜ともなると・〜ともなれば
when it comes to (a higher level), once you're
Noun + ともなると / ともなれば
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ともなると means that once you reach a certain level, age, position or time, things naturally change: 大学生ともなると、自分で考えて行動しなければならない, "once you're a university student, you have to think and act for yourself".
The noun is a stage that's a step up: 大学生, 社長, プロ, or a peak time like 十二月 or 週末. The second half is what is naturally expected at that level: more responsibility, higher standards, crowds, a festive mood.
ともなれば means the same. Plain となると, "when it comes to", is close, but ともなると adds a sense of "at such an elevated level".
Compare ともあろう (N1), which criticises someone for failing to live up to their status.
Examples
大学生ともなると、自分で考えて行動しなければならない。
Once you're a university student, you have to think and act for yourself.
社長ともなれば、責任も重い。
When you're company president, the responsibility is heavy.
十二月ともなると、街はクリスマス一色になる。
Come December, the whole town turns Christmassy.
プロともなれば、ミスは許されない。
At the professional level, mistakes aren't allowed.
週末ともなると、この公園は家族連れでいっぱいだ。
At the weekend, this park is full of families.