def age_vocab(max_age, mon=1, symbol=None):

    age2idx = {}

    idx2age = {}

    if symbol is None:

        symbol = ['PAD', 'UNK']

    for i in range(len(symbol)):

        age2idx[str(symbol[i])] = i

        idx2age[i] = str(symbol[i])

    if mon == 12:

        for i in range(max_age):

            age2idx[str(i)] = len(symbol) + i

            idx2age[len(symbol) + i] = str(i)

    elif mon == 1:

        for i in range(max_age * 12):

            age2idx[str(i)] = len(symbol) + i

            idx2age[len(symbol) + i] = str(i)

    else:

        age2idx = None

        idx2age = None

    return age2idx, idx2age