Translate the following question to SQL: How many heads of the departments are older than 56 ?

Schema: CREATE TABLE department (
    Department_ID INT   PRIMARY KEY,
    Name TEXT,
    Creation TEXT,
    Ranking INT,
    Budget_in_Billions REAL,
    Num_Employees REAL,
    UNIQUE (Department_ID)
);

CREATE TABLE head (
    head_ID INT   PRIMARY KEY,
    name TEXT,
    born_state TEXT,
    age REAL,
    UNIQUE (head_ID)
);

CREATE TABLE management (
    department_ID INT   PRIMARY KEY,
    head_ID INT   PRIMARY KEY,
    temporary_acting TEXT,
    PRIMARY KEY (department_ID, head_ID),
    UNIQUE (department_ID, head_ID),
    FOREIGN KEY (head_ID) REFERENCES head(head_ID),
    FOREIGN KEY (department_ID) REFERENCES department(Department_ID)
);
user
SELECT count(*) FROM head WHERE age  >  56
assistant