She's the country's youngest Prime Minister, he's his country's second youngest.

Both were named on Time's list of 100 most influential people of 2018.

Donald Trump even thought they were husband and wife at the Asean summit last year.

Jacinda Ardern, 37, and Justin Trudeau, 46, appear to "have a warm friendship and shared political ideals", the UK's Guardian has declared.

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Prime Minister Jacinda Ardern talks to Canadian PM Justin Trudeau as they arrive for the the second day of the Commonwealth Heads of Government Meeting at Windsor Castle. Photo / AP
Prime Minister Jacinda Ardern talks to Canadian PM Justin Trudeau as they arrive for the the second day of the Commonwealth Heads of Government Meeting at Windsor Castle. Photo / AP

The pair have become favourites of the European press during their stint at the Commonwealth Heads of Government Meeting.

They were snapped overnight walking into Windsor Castle together for the leaders retreat.

Trudeau listened intently as Ardern spoke, with his jacked slung over his shoulder and black aviator sunglasses on.

Earlier this week, the pair took a selfie together, along with London City Mayor Sadiq Khan, at City Hall where they spoke to school children.

While in France, they met between speaking at the Sciences Po university.

Ardern was asked by Time to write about Trudeau after the pair were named on the top 100 list.

She said Trudeau's name would be etched in the history books for shifting the political landscape at such a young age.

"Youth alone is not remarkable, but winning over people with a message of hope and warmth, tolerance and inclusion, when other politicians the world over choose an easier route – that is remarkable," Ardern wrote.

Ardern's partner Clarke Gayford is are also in Europe for the event, with Gayford attended a spouses programme which included a boat trip down the Thames and lunch at Somerset House.

Earlier this week, Canada PM Justin Trudeau took a selfie with PM Jacinda Ardern and London Mayor Sadiq Khan, at City Hall where they spoke to school children. Photo / AP
Earlier this week, Canada PM Justin Trudeau took a selfie with PM Jacinda Ardern and London Mayor Sadiq Khan, at City Hall where they spoke to school children. Photo / AP

Gayford's time will soon be taken up by full time baby duties, with Ardern expecting her first child in June.

The leaders of the Commonwealth met for a day long retreat at Windsor Castle overnight and granted the Queen's wish for Prince Charles to take over the role, which is not hereditary.

In a statement, the leaders said they recognised the role of the Queen in championing the Commonwealth and named Charles as the next head.

Ardern had publicly supported the hand over to Prince Charles in advance of the meeting.

"I get a real sense from the interactions I've had that his understanding of some of the challenges we've had in the Commonwealth is excellent, particularly around environmental issues which will serve us well."

Trudeau had also backed the move.

Ardern said climate change was also discussed by the leaders and she believed there was a sense of urgency building over that, partly because of the number of small island states in the group.

"Today I did hear one of the leaders at the meeting say they had not considered the extent of some of these international issues such as climate change, the extent to which they would be impacting on islands in the Pacific.

"It really did shift some of the sentiment."

Ardern takes a video call from Trudeau today in October. Photo / Supplied
Ardern takes a video call from Trudeau today in October. Photo / Supplied

The leaders have agreed on an Oceans Charter to protect the oceans and resources in them.

Ardern said New Zealand would be involved in work to address ocean acidification for its part of that.

Ardern also called for leaders to consider a CPTPP-style trade deal across the Commonwealth - an idea championed in the past by British Foreign Minister Boris Johnson as part of the Brexit campaign, and Foreign Minister Winston Peters.

Ardern said there was renewed interest in issues of trade given the protectionist moves elsewhere, such as the US.

"That tends to be at the expense of small states such as ourselves but also like many of those represented in the Commonwealth."