def age_bracket(x):
    """
    Converts ages from [15,65] into age brackets (1,5). If age
    is out of that range, returns None

    Integer -> String

    """
    if 15 <= x < 25:
        return '1'
    elif 25 <= x < 35:
        return '2'
    elif 35 <= x < 45:
        return '3'
    elif 45 <= x < 55:
        return '4'
    elif 55 <= x < 65:
        return '5'
    else:
        return None