partner-contact/partner_contact_birthdate/views/res_partner.xml

<?xml version="1.0" encoding="utf-8" ?>
<!-- License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html).
     Copyright 2015 Grupo ESOC Ingeniería de Servicios, S.L.U.
     Copyright 2019-2020: Druidoo (<https://www.druidoo.io>)
-->
<odoo>
    <record id="view_partner_form" model="ir.ui.view">
        <field name="name">Birthdate Date field</field>
        <field name="model">res.partner</field>
        <field
            name="inherit_id"
            ref="partner_contact_personal_information_page.personal_information"
        />
        <field name="arch" type="xml">
            <xpath expr="//group[@name='personal_information_group']">
                <field name='birthdate_date' />
                <field
                    name="age"
                    attrs="{'invisible': [('birthdate_date', '=', False)]}"
                />
            </xpath>
        </field>
    </record>
</odoo>