（政府は某ワクチンが）どの年齢まで効果的か検証しているって英語でなんて言うの？

It is said ~ ~と言われている the effectiveness of each of the vaccines currently available in the market 現在市場で使われているそれぞれのワクチンの効果は、 varies in the age groups　年齢層で変化している。 It is said that the effectiveness of each of the vaccines currently available in the market varies in the age groups. それを今はまだ検証段階。3つの例を簡単に表せば、 It is still under investigation. It is still under testing. It is still under examination.

The government is evaluating how effective a certain vaccine is across different age groups. The government is studying up to what age the vaccine remains effective. 自然なのは across different age groups（いろんな年齢層で） by age（年齢別に） how it varies depending on age（年齢でどう変わるか） のように言う言い方です。