Does anyone know how old the characters of Meredith and Christina, are supposed to be?

Uuuhm...In early season 2 is Cristina having a surgery and on the OR schedule is written that she´s 28.
In s3 (when Ellis is suddenly lucid) Meredith says to her that she was in Europe 5 years ago and in s5 she says to Sadie that they´re no longer 25 sleeping on their way through Europe. So my conlusion is - Meredith´s 30 in s3 which means she´s 29 when she starts her internship and 31 in late season 5. Lexie says to Mark in s5 that she´s 24. Thatcher left Meredith when she was 5 and age difference between Mer and Lexie is 7 years, so I think it could fit.
Izzie´s 29 in s5 (but she was having a birthday shortly before it, remember), but I have no idea how old George or Alex are.
Meredith wasin Europe before med school, Cristina was on 2 other colleges, Izzie took 6 years to graduate, Alex didn´t have very good results at the college so George seems to be the only person that started and finished med school in regular age and really, for me he seems to be the youngest. Alex could be somewhere around Izzie´s age or maybe older???

Ok, that was quite exhausting :D Hope you understand what I was trying to say :D

Lexie is a second year intern, and she's a year younger than everyone at 26. Which would lean towards Meredith and Christina being 29 or 30.

I have no idea! I wish I knew. I think they are somewhere from like 27-30.
I think Izzie mentioned it one time that she was either 29 or 27, I can't remember.
It's in the episode where she is having the interns work on "patient x"

i believe that meredith and christina are 29 to 30... :)

Well Goerge Died at age 29 as the nurse says at the final epi of S5.
Mer was in Europe 5 years ago when she was 25 so guess she's around 30-31.
Lexie is 24 as she tells goerge.
dunno abt the rest.

in the original script i think Meredith Grey is 32 i think Derek is born in 1965 i think He is late 30s Izzie is born in 1979 she is 25 George is born in 1979 he is 25 as the same age like Izzie Cristina is born in 1973 she is 31