The following copies a given birthdate into a new constant b18 and varies it by adding 18 years to it. The comparison with the current date then shows whether the person is 18 years old or not.

// const b contains birth date to be tested
document.querySelector("input").addEventListener("input",function(){

 const b=new Date(this.value);

 const b18=new Date(b.getTime());
 b18.setYear(b.getFullYear()+18);

 // alternatively you could calculate that "critical birthdate", 
 // 18 years ago, that makes you an adult today:
 const d18=new Date();
 d18.setYear(d18.getFullYear()-18);

 console.log(new Date()>b18,d18>b);
})

<input type ="date" value="2003-08-29">

You can do this:

        <input id="age" type="number" />

        let age = document.getElementById('age');
        let minAge = 18;

        age.onchange = () => {
            if(!age.value >= minAge){
                    age.style.border = "1px solid red";
                } else if(age.value >= minAge) {
                    ​age.style.border = "1px solid green";
                }
        }
 ​