Now aged 74 himself, it is not possible for the King to surpass that milestone, however his time on the throne is sure to be a memorable one as he leads the monarchy into a new age.
Charles and Queen Camilla have marked the day privately and quietly at the Balmoral estate in Scotland, as the King reflects on the loss of the Queen and his first year as monarch.
Royal experts have weighed in what Charles has done well so far, overseeing a “peaceful and political” transition of power.
Camilla too was also praised for her steadfast companionship and support for the King over the past two decades of their relationship.
Royal commentator Richard Fitzwilliams told Express.co.uk: “The overall effect [of his reign] has been positive, and symbolic of the continuity which monarchy, as a symbol of national unity, must give to the nation.
“Camilla benefitted enormously from the Queen’s request that she be made Queen Consort. She has been an ideal companion for Charles. He has shown that he appreciates the difference between this and his previous role where he was an activist.”
He acknowledged some of the King’s best decisions, including appointing more Counsellors of State to ensure former working royals Prince Harry and Prince Andrew are never called on to take up more royal duties.
Charles has also begun looking into the monarchy’s troubling historical links with the slave trade, and has been open in his remorse for the institution’s involvement.
Mr Fitzwilliams also gave an insight in what we could expect next from the King and Queen, including “more royal visits abroad, especially to Commonwealth countries”.
Charles and Camilla are set to fly to Kenya in the autumn for their first official visit to the Commonwealth in their reign, and are also planning to visit Canada and Australia.
Royal expert Kinsey Schofield said of the King: “His first year has been peaceful despite political chaos. I think the King has sacrificed a significant amount of his life for this responsibility.
“I think he is humbled by the blessing that is his duty. He will be an excellent King because his mother was one of his greatest mentors.”
However it has not all been plain sailing for the King, as a recent poll suggests members of the Commonwealth are less in support of him than they were of Queen Elizabeth.
Just 47 percent of respondents in countries that recognise King Charles as head of state would vote to keep him there, according to polling by Lord Ashcroft.
As of 2023 King Charles is the head is state of just 15 Commonwealth realms: Antigua and Barbuda, Australia, the Bahamas, Belize, Canada, Grenada, Jamaica, New Zealand, Papua New Guinea, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, the Solomon Islands, Tuvalu and the UK – five others have their own monarchies and the rest are republics.
According to Lord Ashcroft, if a referendum were held tomorrow, the republican vote would win out in six of these countries, notably in both Australia and Canada.