On your older cases you should seperate them into the different manufacutres and if they are all the same manufacture then there could be a slight difference in case capacity.  As far a the difference is velocity between the rounds that could be something as simple as the powder that you are using or the case neck thickness from your used cases.  If you want to try a little experment get youself a case neck turning tool and get all case necks to the same thickness then try shooting them and see what happens.  A thicker neck could hold the bullet a little bit tighter and create a little bit more pressure witch would give you a higher velocity out of that round. 

I have found that for a hunting round I will use only once fired cases out of the same lot witch means that they should of all been bought at the same time and hope.  Or new unfired cases witch are not that expensive if you buy them in 100 case lots.  Then use the once fired  cases you have for paractice rounds.

I would not worry at all about the age of the cases. I'm using FA49 cases in my 30-06. In fact I still have some with the origional military primer;s in them and they work fine.

I have some cast loads for that 30-06 that don't cronograph within 100fps shot to shot. It's a real joke but they shoot great! A lot of guy's make a lot of to-do about using a cronograph. But After using one for over 15yrs, the best thing I find about it is that I know how fast my loads are and, more important, mine will read out trajectory and velocity for me to ranges well beyond what I will shoot. I'll also tell you if your matching factory ballistics, assuming you cronograph the factory stuff you want to match in the same rifle. That would give you a reasonable expectation of pressure in that it's probably about the same as factory. The glicth there is that most factory ammo is loaded with different powder's than avaaliable to the handloader. They may be reaching the same pressure with different curves that make the load safe whereas with your powder the load may not be.

All good stuff to sit around and argue about but nothing will be solved. The bottom line is does your load shoot well for you and does it have adequate velocity for what you want. After that it's all a bunch of nit picking. I love my cronograph, but not for all the reason's some guy's claim. I can remember the days I didn'/t have one. It was great, my bullet's were going as fast as I wanted to say they were going and nobody could say different. But I also had to shoot a lot to figure out trajectory tables. A lot more bother but a lot more fun also! The end result was that animals properly hit with proper bullet's, still expired properly!

We usually get 5 reloads out of our casings without any signs of wear or performance issues.  I couldn't afford to shoot casings once and delegate them to practice. I guess you could say I get my moneys worth.

Brass is better than most other metals at resisting corrosion and deterioration.  Sure, it may patina and brown on you, but unless it's exposed to harsh chemicals or corrosion it should have a shelf life of indefinite.  Most other metals are the same as long as they are protected from usage or corrosion.  The ore that these metals are smelted from is typically ancient in age anyway. 

Your finished ammo once loaded should have a shelf life of several decades as long as it's stored correctly in a fairly stable environment away from extreams in temerature and humidity fluxuation.