script.js

let userInput = document.getElementById('date');
userInput.max = new Date().toISOString().split('T')[0];//this will prevent to select future dates
let result = document.getElementById('result');

function calculateAge(){
    let birthDate = new Date(userInput.value);

    let d1 = birthDate.getDate();
    let m1 = birthDate.getMonth()+1;//start with '1' because it start with 0
    let y1 = birthDate.getFullYear();

    let today = new Date();

    let d2 = today.getDate();
    let m2 = today.getMonth()+1;//start with '1' because it start with 0
    let y2 = today.getFullYear();

    let d3,m3,y3;
    //this is for Year
    y3 = y2-y1;

    //for the month
    if(m2 >= m1){
        m3 = m2-m1;
    }
    else{
        y3--;
        m3 = 12 + m2-m1;
    }

    //for the days
    if(d2 >= d1){
        d3 = d2-d1
    }
    else{
        m3--;
        d3 = getDaysInMonth(y1,m1) + d2-d1;
    }
    if(m3 < 0){
        m3 = 11;
        y3--;
    }
    result.innerHTML = `You are <span> ${y3} </span> years, <span> ${m3} </span> months and <span> ${d3} </span> days old.`
}

//this will find the exact days
function getDaysInMonth(year,month){
    return new Date(year,month,0).getDate();
}