The Downton Abbey Series 1 script says Mary was 21 in 1912 (and thus born in 1891), that Edith was 20 (thus born in 1982) and that Sybil was 17 (thus born in 1895)
Julian Fellowes announced that in 1912, Mary Crawley was 21, Edith 20, and Sybil 17. Sybil was 24 when she died, making Mary 28, and Edith 27. Mary had her baby at age 29.
She had herbaby at 30; George was born in 1921, making Mary 30 at the time.
Laura Carmichael (who plays Lady Edith) says Edith was born in 1894. I've always thought Mary was two years older and Sybil two years younger than Edith.
We used that originally, but per canon policy, actor thoughts are not included. The script - written by Jullian Fellowes - explicitly states that Edith is "20" in 1912. Thus, she was born in 1892.
If Mary is 30 in 1921 that means she would be 80 in 1971. What a different world would she see if she lives past 80! A flashforward into the future would be awesome at the end of season 6. Don't you think? The surviving characters would find themselves in very different England.
What do you think?