Question about Minerva McGonagall

[Blaise]

No: HPFGUIDX 18714

Hi everyone,

Been lurking for a long time due to university and life catching up 
with me, but I've got a question which I hope someone can answer or 
point me in the correct direction for an answer.  I apologise in 
advance if it's something that's already been discussed to death...

Has JKR said anywhere, e.g. in a chat/interview, how old Minerva 
McGonagall is?  I'm writing a fanfic about her, and I don't want to 
get things too badly wrong.  

Thanks

Blaise.