Platinum Jubilee: How old was the Queen when she came to the throne?

Queen Elizabeth II is the UK's longest-reigning monarch and, as the country celebrates her Platinum Jubilee this weekend, the vast majority of her subjects have never known another sovereign.

She ascended to the throne at the age of just 25 after a series of events which few would have predicted left her to be first in line.

Born Elizabeth Alexandra Mary on 21 April, 1926, 'Lilibet' was destined for marriage, not the throne.

She and her younger sister Margaret enjoyed their earlier years without the pressures of either being heir apparent.

Their parents split their time between London and the family's home on the grounds of Windsor Great Park, with Elizabeth and Margaret were educated at home by tutors.

Why did Elizabeth become queen?

When her childless uncle Edward VIII abdicated in 1936, so he could marry divorcee Wallis Simpson, Elizabeth's father inherited the crown as George VI and, with no sons, the role of heir fell to Elizabeth.

The turn of events has led to her title as 'the Accidental Queen', though nobody would doubt her dedication to a role she carried out for longer than any other monarch.

Read more: Platinum Jubilee concert: Who's performing and can I get tickets?

When she and sister Margaret had to move to Buckingham Palace, Elizabeth is said to have asked her nanny: "What, you mean forever?"

But the magnitude of the task ahead of her didn't phase the future monarch, who on her 21st birthday vowed to spend her life serving her country.

When did the Queen marry Prince Philip?

That same year she married Prince Philip, a distant cousin of hers, who was 26 at the time and renounced his titles as prince of Greece and Denmark, as well as a career in the Royal Navy, to be with her.

She later described him as "my strength and stay" — something that would become invaluable to the young couple as their life quickly became unrecognisable from what they may have imagined.

Read more: In pictures: 70 years of the Queen on the balcony of Buckingham Palace

As heir to the throne, Elizabeth started taking on some responsibilities, including making radio broadcasts from Windsor Castle in 1940, following the outbreak of the Second World War the previous year.

In the speech, the 14-year-old princess reassured evacuated children that "in the end, all will be well; for God will care for us and give us victory and peace".

As colonel-in-chief of the Grenadier Guards, Elizabeth made her first public appearance inspecting the troops in 1942 and also began to accompany her parents on official visits within Britain — and in 1945 she joined the Auxiliary Territorial Service to help in the war effort.

Elizabeth could never have imagined how soon she would have to step up to lead the nation.

On 6 February 1952, Elizabeth and Philip were in Kenya, staying in an isolated lodge, when they were told that her father, King George VI, had died at the age of 56.

At that moment, her whole life changed and she became Queen Elizabeth II.

The tour was cut short and Elizabeth returned home to take up her new role.

How old was Elizabeth when she became queen?

Aged just 25, with two young children — Charles and Anne — she and her family were thrust into the spotlight as she became the British monarch.

At the time, Winston Churchill was said to have wept, describing the Queen as "only a child", but she went on to prove him and other doubters wrong.

Despite her young age, she was quoted as saying: "I no longer feel anxious or worried. I don’t know what it is – but I have lost all my timidity somehow in becoming the sovereign and having to receive the prime minister."

Such an attitude went on to be echoed by Churchill, who said: "All the film people in the world, if they had scoured the globe, could not have found anyone so suited to the part."

When was the Queen's coronation?

The Queen's coronation came two months later on 2 June, 1953 in a ceremony at London's Westminster Abbey.

The day coincided with news of Edmund Hillary’s conquest of Everest, another historic moment, but it was the coronation that marked a milestone in the world of television.

According to the BBC, more than 20 million people watched the coronation service on television, outnumbering the radio audience for the first time.

The broadcaster later said: "The BBC knew the event would be popular—based on the reaction to the limited broadcast of George VI's coronation procession—but could not foresee that it would mark the coming of age of television, as well as the modernisation of the monarchy."

Britain's longest-serving monarch

Since that momentous occasion, which marked the start of what is now nearly seven decades of rule as monarch, the Queen's reign has seen countless historic moments.

Elizabeth, who went on to have two more children, Andrew and Edward, is now Britain's longest-serving monarch with a reign that has taken the country from the post-war era into the 21st Century.

In September 2015 she passed her great-great-grandmother Queen Victoria, who held the throne for 63 years and 216 days, as the longest-serving monarch.

No mean feat for someone dubbed the 'Accidental Queen'.