、。
Interest in baseball cuts across all classes and ages in our country.
Literal
Baseball [to-への] interest [topic-は] our country [in-では] all class, age [to-にわたっている].
~にわたる/にわたっている ('extend over, span') is used for things that extend across a range — time periods, territories, categories, generations. ~への marks a directed attitude/relation toward a target. わが国 is the formal/literary 'our country, my country,' common in official speeches and writing. The 階層 ('stratum, class') + 年齢 ('age') combination describes the demographic spread.