Loading .txt file content and filename into pandas dataframe
As mention in the comments, pandas work really really well with csv so if you are generating the data your self you might consider to save the data in csv format.
allFiles is just used once, dont define it; use glob in loop instead.
Replace stories and filenames with just one DataFrame, and use pandas.concat()
If you are just updating the script evertime you run it, you can just have a age variable.
Never use file_[0:-4] to remove filextensions, use os.path.splitext.
I guess you will run this code for a lot of diffrent ages, so make a function out of it.




from os.path import basename, splitext
import pandas as pd

def getDataByAge(age)
    res = pd.DataFrame()
    for file_ in glob.glob(".../test_age%d/*.txt" % (age)):
        with open(file_) as f:
            textf = " ".join(line.strip() for line in f)
        res = pd.concat([res,pd.DataFrame(data = {"filename" : [splitext(basename(file_))[0]], "stories" : [textf], "age" : [age]})])
    return res