This is not surprising.
The systolic hypertension in the aged results from arteriosclerosis and gradual decrease of the arterial wall compliance.
Therefore I believe the early intervention and good control of the hyperlipidemia and blood pressure would be essential
in preventing vascular dementia in late life, in 70's, 80's, 90's and even 100's.

"Conclusion: BP is positively associated with risk of vascular dementia"...

Am I dumb in English? Or is it the English researchers?
So, what BP? Aren'r we all living with BP?
I have not seen anyone who doesn't have BP.
The article does not say whether it is low BP or high BP.

..."The association between usual systolic BP and risk of vascular dementia decreased with age (hazard ratio per 20 mm Hg higher systolic BP, 1.62; 95% confidence interval, 1.13-2.35 at 30-50 years; 1.26, 1.18-1.35 at 51-70 years; 0.97, 0.92-1.03 at 71-90 years; P trend =0.006). Usual systolic BP remained predictive of vascular dementia after accounting for effect mediation by stroke and transient ischemic attack. In the population-based cohort, prior systolic BP was predictive of 5-year risk of dementia with no evidence of negative association at older ages." ...
The above is from the text.
Within the range of blood pressure in the population studied, the hazard for vascular dementia increased "per 20 mmHg higher systolic BP" for age groups, 30-50, and 51--70, but not in 71-90.

I agree with WM that perhaps these British researchers could have summarized the results in such a way we can understand more easily. Our WM and I are always available if they wish to consult with us, I believe (a joke)

I was wondering why not in elderly age group, 71-90.
Perhaps I thought by that age the brain vessel damage has become irreversible, and many of these elderly people are already taking
multiple antihypertensives so that the systolic BP may have lost the power of prediction.