1. She's five years his junior.她比他小五岁。

2. She's his junior by five years.她比他小五岁。

   【解析】be two years / ten years etc sb's junior to be younger than someone else by a particular amount 比某人小2岁 / 10岁等：Diane is 12 years her husban's junior. 戴安娜比她丈夫小12岁。（摘自《麦克米伦高阶英汉双解词典》junior）

    be + 基数词 + year/years + sb's junior 和 be + sb's junior by + 基数词 + year/years 都是习语，意思都是“小某人......岁”、“比某人小......岁”，根据基数词决定用 year 或 years 。如：She's four years his junior.她比他小四岁。

She's his junior by four years.她比他小四岁。（摘自《牛津高阶英汉双解词典》第7版）