If you are over 60 years old, you are eligible for a Senior Railcard. 
At the time of publication, this Railcard costs £24.

What does it entitle you to?
You can save 1/3 on standard and first class rail fares throughout Great Britain for 12 months.

You can use it almost anytime during the week, weekends and Public Holidays. 
The only restriction is morning peak period services, Monday to Friday (not including Public Holidays) when journeys are made wholly within the London and South East area.

How can you apply?
Buy online: http://www.senior-railcard.co.uk/
or
Buy from a staffed station ticket office

When applying for the first time you will need to provide proof of age in the form of a passport, driving license, or birth certificate.

National Rail Enquiries
Tel: 08457 48 49 50