Age 21 is a curious juncture. Unlike in parts of the US, it is not the age at which you can first drink in the UK. Nor, as in some countries, is it the age at which you can first vote. For most, the only significant thing about a 21st birthday is receiving a card from a sentimental relation in the shape of an oversized key.
In fact, the only current legal benefits of reaching what used to be defined as the age of majority are being able to: be elected as a member of parliament, local councillor or mayor; adopt a child; hold an airline transport pilot’s licence for an aeroplane, helicopter and gyroplane; and apply for a provisional licence to drive a large passenger vehicle or heavy goods vehicle.