Okay, had a look at the eastenders website.
However, we know that Kat was 13 when she got pregnant, so therefore she would only have been at the most 14 when she had Zoe, so therefore
Zoe = 21
Kat = 35 D.O.B.: 14.01.1970
We don't know anything about Viv, other than she was married to Charlie Slater in 1967. So therefore she must have been at least 18 in 1967 ( it could be 16 by special licence, but im trying to keep this as reasonable as possible). This would mean that
Viv = about 55
Which is reasonable when you remember the history repeating itself storyline when Kat told Zoe that she was her mum. Kat said that her mum (viv) looked after Zoe when she was born because she didn't want the baby to ruin her life like it had her nan's (Mo) Which wouldn't make it impractical that she was 13/14 when she had Viv.
Mo's birthdate is D.O.B.: 18.02.1936
Charlie : D.O.B.: 13.05.1941