Verified answer

２０代では、むしろいい成績よりあなたの夢のために生きて。

２０代ではsounds a bit awkward to me

２０代はis better.

むしろいい成績よりあなたの夢のために

this is not correct.

"(for) A rather than B"

you need to say B「よりむしろ」A (のために)

いい成績より(むしろ)あなたの夢のために

むしろ is not always necessary.

it often sounds better without it in informal sitations

２０代は良い成績より夢のために生きて。

other variations

２０代は成績なんかより夢のために生きて。

２０代は良い成績を取ることより、夢のために生きて。

二十歳になるの時はmakes no sense what so ever.

二十歳means 20 years old.

２０代means 20's.

あなたの夢のために生きてmeans live for your dreams so you did get that bit right.

The grading system in Japan doesn't work as A,B,C,D,F. It works as 1,2,3,4,5. But it's different for each school. It may only go up to 3. Therefore no one would really know what you're talking about.

Maybe you would wanted to say, 二十歳になったら、むしろ　A+ より、あなたの夢のために生きて。

>２０代では、むしろいい成績よりあなたの夢のために生きて。

I think this is the most accurate.