python lambda表达式对象的使用

下面这段代码 ,st是怎么和student关联在一起的

class Student:
    def __init__(self, name, grade, age):
        self.name = name
        self.grade = grade
        self.age = age

    def __repr__(self):
        return repr((self.name, self.grade, self.age))


student_objects = [
    Student('john', 'A', 15),
    Student('jane', 'B', 12),
    Student('dave', 'B', 10),
    Student('jim', 'B', 12),
]
sorted(student_objects, key=lambda st: st.age)

这个lambda以st为参数，获取st的age呀，那句代码意思就是说按照student的年龄排序嘛