- Rose - 19 to unknown.
- Martha - 24/25 to unknown
- Donna - unknown, but probably around 39
- Amy - 21 to 31
- Rory - 21 to 31
- Clara - 27 to unknown
- Bill - 26 approx
I'll give this my best shot. A lot of this is lifted from The Tardis Data Core, which is reliable thanks to the fan's fanaticism with keeping it accurate.
The Doctor twice states on-screen that Rose is nineteen years old (The Unquiet Dead, Dalek) It is later established she left with the Doctor on 6 March 2005. There is no information to indicate how long she travelled with him, although it is probably longer than the two years we saw.
DOCTOR: Two thousand and twelve.
(He looks at a display case.)
ROSE: God, that's so close. So I should be twenty six.
Martha's age or her date of birth are not mentioned on screen. A prose story suggests she was born in 1986, (The Frozen Wastes), but this is probably not canonical.
However, she is a medical student when she meets the Doctor, which would put her at around 24 or 25 at that time, assuming she had progressed normally through medical school.
In the U.K., for a hospital doctor, you have to do a 5 year degree, 2 foundation years, then perhaps another 5 years training depending on your speciality area. If we assume she started training when she was 18, and she is doing her foundation years when we meet her in the show, she'd be 24 or 25.
As with Rose, we don't know how long she actually travelled with the Doctor.
No information about Donna's age is given in any of her episodes. We can only speculate that she was the same age as the actor who portrayed her.
Katherine Tate was born in 1968, and her season was the 2008 season, making her 39-40 at that time.
In The Power of Three that Amy mentions that they've been travelling with the Doctor for 10 years at that point.
AMY: We think it's been ten years. Not for you or Earth, but for us. Ten years older. Ten years of you, on and off.
Amy was 21 when she started travelling with the Doctor (established in The Eleventh Hour), so that would make her 31 in Power of Three.
Rory is a school-mate of Amy; they are in school (primary and high school) together in Let's Kill Hitler, putting his age within a few months of Amy's.
For Clara I'm not including her splinters from Asylum of the Daleks, or from the Snowmen. This is contemporary Clara, who actually travelled with the Doctor in the TARDIS.
Clara Oswald was born on 23 November 1986, as stated in Death in Heaven
CYBERMAN: Correct. You are Clara Oswald.
CLARA: Oh, seriously, this is getting old. Look, there is no Clara Oswald. I invented her. I made her up.
CYBERMAN: Born 23rd November, 1986.
The Clara who was the primary companion met the Doctor in 2013 (more or less), making her about 27 at that time. Note that she moves from job to job (always in a teaching role), so by the time she leaves, several years have passed.
According to the Doctor's sonic sunglasses, Bill was 26 years old by the time of The Pyramid at the End of the World. This, along with other biological information, was seen on-screen during that episode.
As with others, we don't know how long she travelled with the Doctor. Certainly after she got shot and became part Cyberman, she spent several years waiting for the Doctor to rescue her.