BeerBorg

Should there be any harm in mixing 4 month old yeast with 1 month old yeast in a starter? I always upbuild my starters and keep some for the next time and I have 2 left and while I have mixed before with no apparent problems noted, is there, technically, anything wrong with doing so? Will the new yeast overtake or hurt the old yeast in some way that could harm the beer?
I couldn't find much online to support good or bad.

Whatever of the two makes the most happy daughter cells (pitch rate/viability) will "win", but if they are the same strain then I don't see how it matters in any way. If different strains the "winner" will be the most dominant in the final beers profile.

That's about it.

That's pretty much what I've read.
Thanks!

BTW, if it was 2 different strains, and then you washed that yeast and re-used it, then you will most likely end up with a totally different proportion in the next starter, and if you wash that, then again a totally different proportion. Once it goes into your fermenter, then over time, it's going to keep favoring the strain that likes your wort conditions / fermentation temps, etc. the best.

They are very similar strains, just 4 months apart.

Here's a tip, taste your starter wort before using it. My 4 month old jar smelled just fine. I tasted it and it had a very slight vinegar tartness to it. My 6 week old version smelled the same but tasted just like unhopped beer. I threw the old one away. Not sure why it had that characteristic but after a few low quality beers, I'm questioning everything and this is yet another one.
I've read it was a good idea to taste test these but never have. Now I always will.

I haven't washed/saved yeast for a while now but that is certainly a good tip to keep in mind.
Good catch on your part, sir.