I have a few old, rotting split rails found at the edge of the woods on my new property.  I used them to edge a raised bed, thinking that anything nasty that was going to leach would have already done so.  Any opinions or knowledge regarding this process? There is no way to know how old they are but I would guess they are certainly older than 5 years given the advanced state of decomposition.

Thinking of picking up some untreated 1 or 2 x12s to replace the old wood.

I don't think I'd waste my time with untreated wood. Tried it a couple years ago thinking I'd get a couple years out of it. After the first season it was rotted. If you are worried about treated wood build your boxes and cover the inside of the wood with poly.

I guess I don't understand the treated wood issues. Of course I did run behind skeeter trucks my whole childhood and still would if I could find one.

There could be a concern about using treated wood for vegetable gardens.
However, there is research out there that shows little impact of the wood on the vegetables.
University of Missouri
Aggie Horticulture (Texas A&M)

I think you will be fine, especially since they are already old.

Of course, I also use treated lumber to build tree houses, sand boxes and such for my grand kids.
Someone pointed out to me that the children could ingest these materials (chemicals).
I thought about it for a while and decided that if any of my grand kids are chewing on the wood,
natural selection probably needs to kick in anyways.

What about brick or block?

Thanks for all the responses! I would be happy to use brick or block, but I was trying to use found/free materials for this bed ;)  I have 4 4x25 beds that I tilled and then mounded topsoil/manure about 6x8 inches deep.  I used the last of the 6 yards of soil mix I ordered to build a small 8x4x1 bed for herbs.  I didn't till this bed so I wanted to stack it pretty deep, thus the need for edging.

matic, how tall are your beds?

Do some research on the new treated lumber. Because it doesn't contain arsenic and vegetables don't store the chemicals in the fruit...........most of the articles I've read said it was ok to use.

As always, YMMV