...when the office is full of people who were born after you started working for the company.
...you realise that, when you started school, WWII, yes, the one with Hitler and all that, was a more recent event (30 years) than the Falklands conflict (35 years) is to today's 5-year olds.
...you remember a man coming round to change the appliances in your house from town gas to North Sea gas.