Age

Age was the length of time that a person has lived or a thing has existed, or been alive. Different species could live out lifespans of various lengths, and reach different ages.

Rose Tyler was said to be have been 19 on a poster when she left Earth with the Ninth Doctor and didn't return for a whole year. (TV: Aliens of London) Arriving in the year 2012, Rose noted that she would be 26 by this time. (TV: Dalek)

Jackie Tyler celebrated her 40th birthday in February 2007. (TV: Rise of the Cybermen) Later that year, she insisted that she was 40 when the Tenth Doctor, attempting to pass her off as Rose to Yvonne Hartman, claimed that she had aged 57 years as a result of looking into the time vortex. (TV: Army of Ghosts)

Owen Harper died for the first time at only 27 years old. (TV: Dead Man Walking)

Reversing age
Seeking to escape the Last Great Time War, travelled to the far future where he used a Chameleon Arch to transform into a human baby which was left on the Silver Devastation. (AUDIO: The Heavenly Paradigm, TV: Utopia)

Looking into the heart of the TARDIS, Blon Fel-Fotch Passameer-Day Slitheen was regressed to an egg, which the Ninth Doctor took to Raxacoricofallapatorian to give her a second chance at life. (TV: Boom Town)

Through his invention of a hypersonic soundwave manipulator, 76 year old Richard Lazarus reversed his physical age, but the process resulted in dormant genes awakening and transforming him into a monster. (TV: The Lazarus Experiment)

Suspending age
In Pete's World, John Lumic invented the Cybermen, created through the conversion of humans to a metal "body that [would] never age or die." (TV: Rise of the Cybermen)

Having been preserved as a face in a pavement slab by the Tenth Doctor, Ursula Blake noted that she would never age. (TV: Love & Monsters)

After being brought back to life in an undead state with the resurrection gauntlet, Owen Harper stopped aging. (TV: Dead Man Walking)

Accelerating age
Through use of technology from the Hypersonic soundwave manipulator incorporated into his laser screwdriver alongside a sample of the Tenth Doctor's biological code from his severed hand, accelerated his physical age by "a hundred years." (TV: The Sound of Drums) A year later, the Master further aged the Doctor. Regeneration capacity suspended and his true age of 900 brought to the surface, the Doctor was reduced him to a small and wizened creature. (TV: Last of the Time Lords)

Martha Jones' physical age was accelerated to her 80s when she was attacked by resurrection gauntlet. The process was reversed when Duroc was defeated by Owen Harper. (TV: Dead Man Walking)

The Doctor's age
The Doctor's age was widely inconsistent. The First Doctor was said to be 450 years old when he regenerated into the Second Doctor. (PROSE: Iceberg) The Seventh Doctor claimed to be 953 years old soon after regenerating, the same age as the Rani, (TV: Time and the Rani) yet by the time of his war incarnation, he claimed to be 400 years younger than his eleventh incarnation, who said he was "1200 and something". (TV: The Day of the Doctor) The Ninth Doctor claimed to be 900 (TV: Aliens of London), with the Tenth Doctor, Eleventh Doctor and Twelfth Doctor all following on from that. The Tenth Doctor claimed that he was 903, 904 and then 906. (TV: Voyage of the Damned, The Day of the Doctor, The End of Time) The Eleventh Doctor claimed that he was 907 a few days after regenerating, then 909, 1103 and then 1200. (TV: Flesh and Stone, The Impossible Astronaut, The Wedding of River Song, A Town Called Mercy) The Twelfth Doctor claimed to be around 2000 on numerous occasions, (TV: Deep Breath, Smile, Extremis) and roughly 1500 years older than his original incarnation. (TV: Twice Upon a Time)

Through his Time Lord ability to regenerate, the Doctor's true age was not reflected in his physical appearance. (TV: Last of the Time Lords)