odoo-contact-addons/partner_contact_majority/models/res_country.py

# -*- coding: utf-8 -*-
# License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl.html).
from odoo import fields, models


class ResCountry(models.Model):
    _inherit = "res.country"

    majority_age = fields.Integer(string="Majority age", default=18)