Another battery question

One reason is that the 'stronger' (those that have a higher fully charged voltage) batteries will constantly be trying to charge the weaker ones - ie, the batteries will tend to equalize. Also, lead acid and AGM batteries have different charge rates, and max charging voltage.
That said, I might be tempted to try it in a carefully monitored situation to see what happens -- until someone reminds me of something that I'm forgetting.

Phancy wrote:
Why not ages?

It's my understanding new batteries have less internal resistance and most of the amperage will come from them at first and that shortens their life

Your Trojans will be just fine with other stuff. I see people doing this kind of thing all the time and 'they' claim it works just fine.

Why not ages?

you should never mix battery types, ages, or sizes.