based on previous make validation for required in reactive forms
this.userCustomForm.get('age').valueChanges.subscribe(val => {
if (condition) {
this.userCustomForm.controls['licenseNo'].setValidators([Validators.required]);
} else {
this.userCustomForm.controls['licenseNo'].clearValidators();
}
this.userCustomForm.controls['licenseNo'].updateValueAndValidity();
});