While most Doctor Who companions over the past 60 years have been young women, the ages of companions actually varies widely. The first several Doctor Who seasons almost always had a teenage girl as one of the main companions, as an audience surrogate when the series was more aimed at children. The actresses, however, were all over 18.
Modern Doctor Who companions trend a little older, but new Doctor Who companion Ruby Sunday will be the youngest yet. Millie Gibson was just 18 years old when she began filming Doctor Who season 14 last year. Though some companions have been younger in fiction, none of the actors have been younger than 19 until now.
35 Susan Foreman
The Doctor's granddaughter, Susan Foreman, appeared to be a teenager when she and the First Doctor appeared in Doctor Who's first episode, "An Unearthly Child." In the "Marco Polo" serial she said she was in her sixteenth year. However, some Doctor Who audio stories and novels suggest that she was actually much older, aging slowly like the Doctor. In "Doctor Who and the Horror of Coal Hill," the Doctor said Susan was 73. In "The Time Lord Letters," he wrote her age as 97 on her school application form. Actress Carole Ann Ford was 23 in the first season.
34 Ian Chesterton
Ian Chesterton was born in 1934 or 1935. This makes him about 28 or 29 when he meets the Doctor in "An Unearthly Child." Ian was portrayed by William Russell, who was 38. Russell returned as Ian in "The Power of the Doctor."
33 Barbara Wright
Barbara Wright was played by Jacqueline Hill. Hill was born in 1929, making her about 34 in the first Doctor Who serial. This age or a little younger makes sense for Barbara, too, putting her around the same age as Ian.
32 Vicki Pallister
Vicki Pallister was one of the Doctor's youngest companions. She debuted in Doctor Who season 2 serial, "The Rescue" and was around 14, according to the novel Byzantium! However, actress Maureen O'Brien was already about 22.
31 Steven Taylor
Steven Taylor ed the TARDIS crew soon after Vicki, in Doctor Who season 2 serial, "The Chase." Steven was portrayed by Peter Purves, who was 26 years old when he first appeared. Steven would have been about the same age as Purves.
30 Polly
After Steven and the Doctor go their separate ways, Polly and Ben Jackson begin traveling in the TARDIS. Polly (whose last name is later given as Wright, but has no relation to Barbara) was portrayed by Anneke Wills. Wills was 25 when she made her Doctor Who debut.
29 Ben Jackson
Polly's partner in time is Ben Jackson, portrayed by Michael Craze. Craze was 24 in Doctor Who season 3 when he made his first appearance. Polly and Ben departed the TARDIS together a year later.
28 Jamie McCrimmon
The Second Doctor picked up Jamie in 18th-century Scotland, making him one of the few companions from the past. Jamie was 22 years old at the time, the same age as actor Frazer Hines. When the Time Lords wiped Jamie's memory in Doctor Who season 6, they de-aged him and dropped him back in his original time period.
27 Victoria Waterfield
Victoria was another young companion of the Doctor's, who was orphaned in her first appearance when the Daleks killed her father. At about 14 or 15 years old, Victoria ed the Doctor and Jamie. Actress Deborah Watling was 19.
26 Zoe Heriot
Zoe was only 16 when she ed the Second Doctor and Jamie in Doctor Who season 4, serial 7, "The Wheel in Space." Actress Wendy Padbury was 21 at the time, however. She departed the series at the same time as Jamie.