When did the retirement pension start ?

The 'Old Age Pension' was introduced in the UK in January 1909. A pension of 5 shillings per week, or 7 shillings and sixpence for a married couple, was payable to a person with an income below £21 a year. The original state pension qualifying age was 70, and was subject to a means test..
www.plsa.co.uk/Policy-and-Research/Topics/State-pension-reform

It was the first step in the Liberal welfare reforms towards the completion of a system of social security, with unemployment and health insurance through the National Insurance Act 1911.

After the Second World War, the National Insurance Act 1946 completed universal coverage of social security. The National Assistance Act 1948 formally abolished the poor law, and gave a minimum income to those not paying National Insurance. The Basic State Pension was also introduced in 1948.

Usedtobeblonde

I remember as a child that it was a very small sum of money but my memory is perhaps playing tricks my recall is £3: 10s. a week.
I have no idea what that would be worth today.
This would have been in the 1940’s.

According to the Bank of England inflation calculator £3 in 1940 would be worth £140 today. The 10s would be worth about another £23

1908.

The idea was copied from Prussia (Germany) , I believe, who reasoned that it would be better if old people were given a sum of money to live on which would not only keep them from extreme poverty in old age but would also benefit the domestic economy by way of its purchasing power.

In a novel a character referred to the Lloyd George pension of 5 shillings a week.

I remember as a child that it was a very small sum of money but my memory is perhaps playing tricks my recall is £3: 10s. a week.
I have no idea what that would be worth today.
This would have been in the 1940’s.

As far as I know the OAP was brought in in Britain just prior to the first World War.