"For children under 10, the difference of age in months has a big influence on the difference in development."
"for children under 10" 「10歳以下の子供」
"the difference of age in months" 「月齢の違い」
"~ has a big influence on 〇〇" 「〜は〇〇に大きく影響します」
"the difference in development" 「発達差」