def get_age_df(df, age_list):
    """Gets a DataFrame and filters it for the purpose of putting specific
    ages on the Concordia plots. Note the DataFrame must have a column named
    'age'.

    Parameters
    ----------
    df : DataFrame
        A dataframe with a column 'age'.
    age_list : list
        A list of ages you want to place on a Concordia plot.

    Returns
    -------
    DataFrame
        A filtered pandas DataFrame.

    Raises
    ------
    None.
    """
    boolean_series = df.age.isin(age_list)
    return df[boolean_series]