I am using odoo14 . i made a custom module .now i want to make a condition where the date of birth should be less than today date. 

thanks in advance

my code

class StudentDetails(models.Model):
    _name = 'std.record'
    _description = 'Student'

    name = fields.Char(string='Name')
    middle_name = fields.Char(string='Middle Name')
    last_name = fields.Char(string='Last Name')
    roll_no = fields.Integer(string='Roll Number')

    _sql_constraints = [
        ('unique_roll_number','unique(roll_no)','This roll number is already used on another student.'), ]
    std_dob = fields.Date(string='Date of Birth')

    image = fields.Binary(string='Image')
    introduction = fields.Text(string="Introduction")
    std_class = fields.Many2one(comodel_name='std.class', string='Student Class')
    subj = fields.Many2many(comodel_name='std.subject', string='Subject')
    std_id = fields.Many2one('std.class', string='Studentid')