How old is River Song?
How old was she when she "died", and how old was she at other points in the series?
River Song was old, having served twelve thousand consecutive life sentences.
As Melody Pond, she was a little girl in 1969 (or 1970) when she first "died" and regenerated. She was around 40 when she "died" again, this time regenerating into River Song. She died sometime in the 51st century, but "the Doctor saved her consciousness digitally to the Library's computer system, granting her a form of theoretical immortality."
Here's a brief timeline of events, giving some indication of her age during the series.
Melody Pond was born in the 52nd century but taken to the 20th century when one month old. She was still a little girl in 1969, and died six months later, regenerating into a new incarnation.
By the late 1990s, she was an adolescent called Mels.
In 2011, Mels went on the TARDIS to 1938 and was shot by Adolf Hitler, and regenerated into River Song. After recuperating in hospital, River went entered Luna University in the year 5123.
Shortly after receiving her doctorate, she was sentenced to, and served, twelve thousand consecutive life sentences in the Stormcage Containment Facility in the 52nd century.
Here's selected snippets from the TARDIS Index File, which goes into even more detail about further episodes:
Melody Pond was born on the asteroid of Demon's Run in the 52nd century after her mother had been abducted from the 21st century. Within a month, she was replaced with a ganger and taken by Madame Kovarian to the 20th century to be raised to kill the Doctor. (DW: Day of the Moon, The Almost People, A Good Man Goes to War) ...
A LITTLE GIRL LOST
As a little girl, she ended up in an orphanage in 1969 America and at a warehouse in Florida. ... (WC: Prequel (The Impossible Astronaut))...
Six months later, in New York City, dying, she regenerated into a new incarnation. (DW: The Impossible Astronaut / Day of the Moon
By the late 1990s, she was an adolescent in Leadworth called Mels. ... (DW: Let's Kill Hitler) ...
BECOMING RIVER SONG
When the TARDIS landed in a local cornfield in 2011, Mels forced her way aboard at gunpoint and crashed it in Adolf Hitler's office in 1938, inadvertently saving Hitler from the Teselecta. When Hitler shot her, she stunned the Doctor and her parents by regenerating into River Song. ... The Doctor took River to the "best hospital in the universe" to recover. ... After recuperating, she entered the Luna University in 5123. ... (DW: Let's Kill Hitler) ...
River was sentenced to twelve thousand consecutive life sentences in the Stormcage Containment Facility in the 52nd century for the Doctor's murder. (DW: The Time of Angels, The Pandorica Opens) She served the sentence to make the Silence believe him dead, but starting with her first night, escaped often to spend time with her husband, then returned to her cell. (DW: First Night) ...
- River's "present day" spanned the 52nd and 51st centuries - in that order. She entered Luna University in 5123 and was in Stormcage by 5145. One of her last Stormcage appearances was in the 51st century, which was also when she died. How the outside world saw this and just when she was released is unknown.
In the episode "Let's Kill Hitler" River states she is 548 years old IIRC. This episode is the only time her age is actually mentioned. As of season 7 of the new Doctor, River will be in a constant state of flux where sometimes she will be older and sometimes she will be younger. Due to the constant time manipulation I think it will be hard to extrapolate her age at time of "death" since they do not say how old she was.
I would remind everyone on here that it is well established that she is not just human since, she was conceived on ty Tardis, she is therefore, just like the Time Lords and can regenerate; so clearly she would have the same lifespan of any Timelord. Additionally, since the doctor doesn't appear to age, neither would she.
As for her age, all we really need to know is how long she lived as River Song since we know that as Melody Pond she was probably 7 or 8 and then she regenerated into Melody (Rory and Amy's friend) as a Todler, and Melody was clearly in her 20's when she regenerated into River in her 20's. So if we take an age of 30 and add it to how long River lives as River we would have her age within a few years.
It's impossible to know how long the sentence she served was since:
She was pardoned at the time of the mission with the weeping angels and,
She stated in one that she had been released because the man she killed never existed (the Doctor wiped all records of his existence).
Without knowing the length she served, there is no way to know how old she was exactly.