Batteries start to go bad the minute they come off of the assemble line.
It's not like they are great, great, great, great, bad.
It's more like they are great, great, good, good, ok, ok, bad.
In other words, much like people, they slowly wear down and wear out over a lifetime. :B
I take old batteries out of my diesels that won't pass load test and put them in my equipment that I run once a year. I can get several more years out of my batteries doing this. :)
If you want to buy a load tester I suggest you buy a newer type electronic model rather than the old carbon bank types. Last time I checked one could get a nice new electronic model for about 75 to a 100 bucks.