It is almost the norm now to be an older dad in Downing Street

Or, more accurately, it was a rare event.

Boris Johnson will become the fourth premier to have a child in more than 170 years, yet he is the third in recent years.

Both Tony Blair and David Cameron had a child while they were still prime minister.

And the man who inhabited Downing Street between them, Gordon Brown, had two very young children when he took office. Prior to Mr Blair and Mr Cameron, the previous prime minister to have a child in office was Lord John Russell, in the mid 1800s.

Mr Johnson is unusual in that he is not yet married to Carrie Symonds, who is 24 years his junior. But the happy couple plan to tie the knot soon.

It is all a far cry from the days of the first Belfast News Letters, in the late 1730s, which regularly reported on Robert Walpole the then prime minister (and the first).

He was then already aged in his 60s, at a time when people often did not make it past their 40s, and his children were already in or approaching middle age.

Some eyebrows have been raised at the Johnson-Symonds announcement, and others have questioned the timing of the revelation.

But many more people will just be happy for the couple.

Mr Johnson, aged 55, is certainly not a young father. But there is talk now about people often now living the ‘100 year life’, due to the huge increases in life expectancy. In that case, people of the prime minister’s current age might, if lucky, only just be past the mid point in their time on this Earth.

Our columnist Alex Kane has written about the joy of having a child even older than Mr Johnson.

We wish the prime minister and Ms Symonds well on this thrilling news for them.