I believe I've also heard that you should put old brood frames on the outside. I chose not to do that though, because like is mentioned, the bees use it for different food storage. To me it just looks nasty. Especially if it's honey they've stored in these brown/black cells. I then see it as "unharvestable" honey.
I've only been a beekeeper for about 18 months, so I have little personal experience. Of course I have some
personal experience, as to the look of brood comb and the choices I make. But I haven't had to toss out brood comb yet because of age.
I personally try to place any comb that's been used as brood comb into the middle of either brood box. I'll then put fresh frames on the outside. They build fresh comb, store some honey in it, and the honey looks "clean and nice". I have accidentally put brood comb on the outside, not thinking, and when they stored honey in it I realized I'm made a bad choice.
That's my take,