**Explanation:**

To determine the relationship between blood pressure and age, we need to consider both the regression equation of blood pressure on age and the regression equation of age on blood pressure. However, the correct answer is option A, which states that we only need to consider the regression equation of blood pressure on age. Let's understand why this is the correct answer.

**Regression Analysis:**
Regression analysis is a statistical technique used to model the relationship between a dependent variable and one or more independent variables. In this case, the dependent variable is blood pressure, and the independent variable is age.

**The Regression Equation:**
The regression equation is a mathematical representation of the relationship between the dependent and independent variables. It allows us to predict the value of the dependent variable based on the values of the independent variable(s). There are two types of regression equations: the regression equation of blood pressure on age and the regression equation of age on blood pressure.

**Regression Equation of Blood Pressure on Age:**
The regression equation of blood pressure on age represents the relationship between blood pressure (dependent variable) and age (independent variable). It allows us to predict the blood pressure based on the age of an individual. This equation helps us understand how blood pressure changes with age.

**Regression Equation of Age on Blood Pressure:**
The regression equation of age on blood pressure represents the relationship between age (dependent variable) and blood pressure (independent variable). It allows us to predict the age based on the blood pressure of an individual. This equation helps us understand how age can be estimated based on blood pressure.

**Choice of Regression Equation:**
In this question, the correct answer is option A, which states that we only need to consider the regression equation of blood pressure on age. This is because the question asks about the dependency of blood pressure on age, not the other way around. We are interested in understanding how blood pressure changes with age, so we need to consider the regression equation that directly models this relationship.

By considering the regression equation of blood pressure on age, we can analyze the relationship between these variables and make predictions about blood pressure based on age. Therefore, option A is the correct answer.