I don’t know how simply I can put this Khandro. The question: “Does she have to declare a 19 year old conviction?” Your answer: “Why tell them, she isn’t under oath”. Not “Why tell them, it’s too old”. Being under oath indicates a need to tell the truth. Your answer suggested to me that you believed it was acceptable to conceal the conviction not because it was too old, but because no oath was involved. However, let’s move on.
My earlier answer was based on the 2014 revision, Arrods. Page 6 of this guidance document was the source:
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/299916/rehabilitation-of-offenders-guidance.pdf
Drink driving convictions have an “effect” for ten years (for the reason I’ve outlined).
However, further investigations lead me to believe that the five year period you quote may well be correct for all driving offences. I cannot find it definitively anywhere but what I have read suggests you are correct. Five or ten years of course makes no difference to the question here as the offence is 19 years old, but thanks for encouraging me to look deeper.