export default function validate(values, errors) {
  const dob = values["dateofbirthfield"]; // your exact field name

  if (dob) {
    const birth = new Date(dob);
    const today = new Date();

    let age = today.getFullYear() - birth.getFullYear();
    const month = today.getMonth() - birth.getMonth();
    const day = today.getDate() - birth.getDate();

    // adjust age if birthday hasn't occurred yet this year
    if (month < 0 || (month === 0 && day < 0)) {
      age -= 1;
    }

    // Apply validation
    if (age < 18) {
      errors["dateofbirthfield"] = "You must be at least 18 years old.";
    }
  }

  return errors;
}