Names.
I always think about my Mum who is Audrey, Grandmothers who were Norah & Edith and Aunts Eileen, Brenda, Barbara & Jean. On the male side I have Robert, Donald, Geoffrey, Edward & David.
I don't need to look far for the birth dates for Edith or Norah as they had to be about 1910 and Audrey had to be in the Thirties and my other aunts had to be Thirties & Forties but the male's names are not datable.
We still have Robert, Edward, David etc in school today but no Audrey, Edith, Norah etc.
Just look at your own families and see how "datable" your names are for the last few generations.
Teddybear