Really old crossties or other creosote-treated timber are OK to use: 1) they've already leached out just about all the creosote they're going to leach out in their life, and 2) creosote is entirely organic, so whatever tiny bit of creosote they MIGHT leach out will biodegrade into the soil.
The main trouble with using newer creosote-treated timber is not so much the leachation of creosote into the soil, but the fumes that it will give off killing the tender vegetation as it comes up. I laid out a 10'x10' tomato bed for my mother with 4 - 6x6x10' that I had at the plant that were about 3 years old at the time. Even at 3 years old they were still giving off enough fumes to kill the young tomato plants. You could barely smell the creosote, but it was still enough to kill the young plants.
I run a creosote treating plant, btw.
I wouldn't use CCA or ACQ treated stuff under any circumstances. They're both waterborne preservatives (dry chemicals dissolved in water) and eventhough it's claimed that the fixation that occurs between the sugars in the wood and the treating chemicals renders the chemicals insoluble in water after the treating process is fully completed, I know enough about the variability of wood to not fully trust that particular assertion. I think at least to some small degree that once soluble in water means always soluble in water.