She met my father at the tender age of 14, and she soon knew that this was the man she wanted to stay with for the rest of her life.

at 是介词，表示"在...时候"或"在...地点"。 the 是定冠词，用于特指某个特定的事物或情况。 tender 本义是"柔软的、温柔的"，在这里引申为"稚嫩的、未成熟的"。 age 指"年龄、年纪"。 of 是介词，表示所属关系或特征。 tender age of 14 是一个固定表达，字面意思是"14岁的稚嫩年纪"，强调当事人当时还很年轻、不成熟。