In the
Shadows on the Walll bible for DS, Liz is supposed to be 56 and Roger is supposed to be 36 at the start of the series in1966, but that was somewhat altered on the actual series. However, as others have pointed out, it wasn't altered to make Liz older, but to make her younger. In ep #267 we see that Liz was born on February 28, 1917 and Roger on September 14, 1925 (which still gives them an age gap that was crucial in their relationship, but not one of 20 years) so at that point in 1967 they're 50 and 41 respectively. (And to my memory there was never anything else on the series that contradicted those ages.) So, it's definitely inconceivable on so many levels that Liz would be romantically involved with Quentin at any point in the 1920s. But then
Angelique's Descent proved that Ms. Parker doesn't give a flying fig about DS canon (despite saying repeatedly that she was completely committed to staying within it) because she completely altered what anyone would consider an extremely significant scene between Barnabas and Angelique that took place on the show during the original 1795/1796 storyline by adding her own dialogue which completely flipped the dynamic between Angelique and Barnabas as it had been established originally!
But then again, anyone who's read that novel knows that the apparent main purpose of it was to hit the reader over the head again and again with how much of a victim Angelique was, so apparently it was canon be damned.