Hi Denny,

Please try to use a for loop in the function. 
Like:

#computing age
@api.depends('birth_date')
def _count_age(self):
    for record in self:
        if record.date_birth is not False:
            record.age = (datetime.today().date() - datetime.strptime(str(record.date_birth),'%Y-%m-%d').date()) // timedelta(days=365)
        else:
            record.age = 0.0

#computing working time
@api.depends('start_work')
# @api.multi
def _long_work(self):
    for record in self:
        if record.start_work:
            years = relativedelta(date.today(), record.start_work).years
            months = relativedelta(date.today(), record.start_work).months
            day = relativedelta(date.today(), record.start_work).days
            record.work_long = str(int(years)) + ' Year ' + str(int(months)) + ' Month ' + str(day) + ' Day'
        else:
            record.work_long = '0 Year 0 Month 0 Day'



Hope it will help you.
Thanks and Vote up please.