who acted old for their age 这一句不懂怎么理解

句子：The study looked at 13-year-olds who acted old for their age by having "cool" behavior, such as early romantic relationships.

这句话的意思是，研究关注了那些在行为上表现出比他们年龄大的13岁孩子，比如早早开始恋爱等"酷"行为。