My_Test_Repo/force-app/main/default/lwc/setterDemoChildComponent/setterDemoChildComponent.js

import { LightningElement,api } from 'lwc';

export default class SetterDemoChildComponent extends LightningElement {
    userDetails
    @api 
    get details(){
        return this.userDetails;
    }
    set details(data){
        let newAge = data.age*2
        this.userDetails = {...data,age:newAge};
        
    }
}