As from December 2013 HMRC have ceased to issue age exemption certificates CA4140 and confirmed that employers do not have to request a CA4140 to stop deducting NIC from their employees.
HMRC advise that employers should use the passport or birth certificate as confirmation to support the non-payment of NICS (PSC recommends you keep a copy). As set out in HMRC's guidelines, there are some circumstances where HMRC will still provide the employer with a confirmation of a person's State Pension Age (e.g. where there may be sensitive information contained within those documents which the employee does not wish to reveal to their employer).
Once the employer has obtained confirmation of the date of birth, they are able to check (if they so wish) when their employee will reach State Pension age by using the government's State Pensions calculator.
Just to confuse matters, the Department for Work and Pensions (DWP) has not withdrawn the application form for CA4140 from their pension pack for new pension claims so an individual can, if they wish, apply for an age exemption certificate directly from the DWP.