In old age arteries carrying blood in the human body become narrow resulting in an increase in the blood pressure. This follows from

According to continuity theorem as $a v=$ constant as area increases velocity decreases. So when blood flows from narrow arteries to wider one, velocity decreases. According to Bernoulli's principle, $$ P+\frac{1}{2} \rho v^2=\text { constant } $$ As velocity decreases, pressure increases. Thus when arteries become narrow, blood pressure increases.