Object.assign value by using Key – JavaScript

I need to assign the below object to a specific object. as a example. how to assign this value object by using Object Key.

let testObject = {
    "001":{
        name:"John"
    },
    "002":{
        name: "Mike",
        age:35
    }
};

let newObject={
    age:35
};

//Output should be like this
//let testObject = {
//   "001":{
//        name:"John"
//    },
//    "002":{
//        name: "Mike",
//        age:35
//    }
//};