How can I give new keys an older look?

Here's a good tutorial: http://treasuresfromtheheartgifts.blogspot.com/2012/07/rust-and-patina-how-to.html

Try a mixture of vinegar and sea salt. Allow the items to soak for 30 minutes. Check and allow more time as needed.

Aluminum or steel----lay them out on plastic, spray with oven cleaner(yellow can) or use gel toilet bowl cleaner(ZEP brand) or just straight Lye(cheap at Acehardware) let sit in solution in sun until they look how you want them(this darkens them/oxidizes) you'll need to sand or buff with steel wool to remove oxidation /highlight them Brass https://inmyownstyle.com/how-to-age-brass-in-less-than-5-minutes.html

Mix steel wool in a jar of vinegar and throw them in over night.

You can paint the entire key with black craft paint, then wipe it off with a damp rag until , you get the patina you are seeking.