[Python] How to calculate a user’s age from a DateField in Flask-WTF?

I don't have know a error message or something like that, so how can I calculate the user’s age from form.birthday.data in Flask so I can display it in a template?

I’m using Flask-WTF for forms. Here’s my code

class RegistrationForm(FlaskForm):
username=StringField('Username',validators=[DataRequired(),Length(max=20)])
email=EmailField('Email',validators=[DataRequired(),Email()])
password=PasswordField('Password',validators=[DataRequired(),Length(min=8)])
password2=PasswordField('password2',validators=[DataRequired(),EqualTo('password')])
birthday=DateField('Birthday',validators=[DataRequired()])
submit=SubmitField('Register')

class LoginForm(FlaskForm):
username=StringField('Username',validators=[DataRequired(),Length(max=20)])
email = EmailField('Email', validators=[DataRequired(), Email()])
password = PasswordField('Password', validators=[DataRequired(), Length(min=8)])
birthday = DateField('Birthday', validators=[DataRequired()])
submit = SubmitField('Register')

@app.route('/register',methods=['GET','POST'])
def register():
form=RegistrationForm()
if form.validate_on_submit():
session['username'] = form.username.data
session['email']=form.email.data
session['password']=form.password.data
session['birthday']=form.birthday.data
u=User(session['username'],session['email'],session['password'],session['birthday'])


this is a unfinished code about registration and login, and if there are any problems or things to change in the code tell it, for the moment I will continue to make a code without an age variable

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