It was never stated explicitly in the book.

Because of the natural attraction between Anathema and Newt, I would assume she is in her early or mid 20s--same as Newt.

Simple enough, right?

Here is the cincher: Adam Young, when he first meets Anathema outside of her home in Jasmine Cottage, he makes a comment about her being old. Given that kids can exaggerate (and the Them are no exception) and that anyone older than 16 might as well be ancient to them, I feel he wouldn't have made a comment unless she was a little bit up there in the numbers age-wise.

While researching this question, I found this in the Wiki (http://wiki.lspace.org/mediawiki/Anathema_Device):

Anathema is a self-sufficient young woman with a Ph.D. degree.

I don't recall that being mentioned in Good Omens (I may have overlooked it) but that would stand to substantiate the claim that she is older.

Is there any information that is available that might be able to shine some light on this?

Thanks.