def age_to_eye_diameter(age):
    """Calulates the size of an eye given an age 

    Args:
         age (float): age of the observer
    Returns:
         D_eye (float): Best diameter of Eyepiece (mm)    
    """ 

    if age <= 20:
        D_eye = 7.5
    elif ((age> 20) and (age<= 30)):
        D_eye = 7
    elif ((age> 30) and (age<= 35)):
        D_eye = 6.5
    elif ((age> 35) and (age<= 45)):
        D_eye = 6
    elif ((age> 45) and (age<= 60)):
        D_eye = 5.5
    else: 
        D_eye = 5.0

    return D_eye