The news has come suddenly and surprisingly, but not entirely shockingly. Anyone who spent teenagehood and beyond doing something they cherished will want to grow up at some point. Smith has come of age through cricket and through captaincy. He has now come out the other side.
Smith led South Africa through their most successful period in Test history. They have been on an unbeaten run away from home for almost eight years and have not lost at home in five years. He has been in charge of the top batsman, top bowler and leading all-rounder, all at the same time. He has taken them to No.1 in the world and kept there for 19 months. He has been their talisman and their frontman, a frontman in every way including the literal, opening the batting in difficult conditions and averaging almost 50 as he approaches the end.
Many people will not understand why the end should come now but the truth is that it has been coming for some time. Almost a month before the day Smith announced his decision to walk away, he gave his first hint that he would do so. He posted a picture on Instagram of his wife and and two young children with the caption. "Always miss these guys when away!..... last one I promise." Now we know what that meant.
That was a firm sign and one that was discussed on these pages at the time but the indications had been building for a while. Two weeks before that, Smith traveled to Ireland to collect the certificate confirming his citizenship to that country. When it was reported, he assured fans he was not turning his back on South Africa. He tweeted: "I love my country and remain always a loyal South African. Half my family is Irish now and visiting them is easier with dual nationality."
Before that, in December 2013, a story sprouted that Smith had threatened to resign the captaincy over selection issues. He killed that suggestion too. In November 2012, Smith signed a three-year deal with Surrey which he could not fulfill in its first because of an ankle injury. He had to cut short his stay in London, where he had relocated with his young family, and expressed disappointment.
Smith's family is ultimately the reason he has reached this point and we would have known that as early as after the 2011 World Cup. That was his last assignment as ODI captain and he earned the wrath of the public when he did not return home from the tournament with the rest of the team but instead went to Ireland to propose to Morgan Deane. An over-sensitive public loathed her then but soon grew to love her as the woman who turned their captain into a softer, nicer and more laid-back person.
With marriage and fatherhood, the other side of Smith emerged. Far from the bubblegum-chewing hard-arse that stared opponents down on the field, Smith allowed his gentle, loving side to come to the fore. The nation grew to adore him in his new role, even when he did not score the runs they wanted.
But cricket is a results-driven game and eventually the lack of positive numbers will catch up. Smith has gone eight innings without a half-century, which is not that many especially considering that less than six months ago he scored a double-hundred against Pakistan in challenging conditions against a challenging attack in the UAE. Still, what Mitchell Johnson has done to him in this series has exposed old weaknesses and it seems Smith has had enough of scar tissue being exposed.
Not for the first time in his career, Smith will exit in ungainly fashion. South Africa are two days away from the end of the series against Australia, a series which they will have to fight hard to draw. Should they manage that, they would extend their unbeaten run to 15 series. If they do not, it will be the first time since March 2009 that they have been defeated.
Either way, it is the end of an era. An era that began and ended in August 2012 when Mark Boucher retired and South Africa became the No.1 ranked Test team. An era that looked more like ending when Jacques Kallis retired after the Boxing Day Test last year. But an era that can be rightly be called the Smith era. It belonged to him and now it's over.
Firdose Moonda is ESPNcricinfo's South Africa correspondent