Jane is in her mid-thirties, but looks twenty-five.
→    제인은 30대이지만25살로 보인다.

[예문]
mid+(시간,나이)= in the middle of +(시간, 나이)

 Jane is in her mid-thirties, but looks twenty-five.
=Jane is in the middle of her thirties, but looks twenty-five.
제인은 30대이지만25살로 보인다.

*early 이른,초반에 =at the beginning of
  late 늦은,후반에  =at the end of