can i charge my iphone 12 with an old charger

Yes.

Also see the following from Buy iPhone 12 and iPhone 12 mini - Apple

As part of our efforts to reach our environmental goals, iPhone 12 and iPhone 12 mini do not include a power adapter or EarPods. Included in the box is a USB‑C to Lightning Cable that supports fast charging and is compatible with USB‑C power adapters and computer ports.

We encourage you to re‑use your current USB‑A to Lightning cables, power adapters, and headphones, which are compatible with these iPhone models. But if you need any new Apple power adapters or headphones, they are available for purchase.

Go for it. As long as you trust the quality of the power source, I don't see why not. Just be wary of junk. I'd worry more about the possibility of a power adapter catching on fire than of damaging an iPhone.