Adjective equivalent of "As old as the hills"

Is there a one-word, more formal adjective for the expression "as old as the hills" ?

The idea is that there is something very old but a little forgotten today (and usually one uses this expression when speaking to someone who doesn't know about the thing).

Example sentences : "This soup is great, you really need to tell us the recipe!" "It's nothing original you know. The recipe is (missing adjective here). The Romans already knew about it."

"There's civil war again in region X, I am a little surprised." "You know, the enmity between ethnic groups Y and Z there is (missing adjective here). We have historical records from ten centuries ago"

A simple one that seems to closely fit your sample sentences is age-old. Admittedly it sounds perhaps a little more natural to say is an age-old one, but is age-old works for me. It doesn't have the negative connotations of some of the "old" words, yet implies that the subject has been around for a long long time. timeworn is in a similar bracket.

Antediluvian literally means "from before the flood."

The flood referred to is The flood which covered the Earth, Noah's Flood, Utnapishtim's flood in the Epic of Gilgamesh. Not very appropriate for a recipe, or gathering.

: [from] before the flood described in the Bible. 2. a : made ... a long time ago "an antediluvian automobile" b : extremely primitive Merriam Webster

Ancient Recipes in age sequence, resourced from the family.

Traditional since 1950 Mother's Marmalade 1945. Grandmothers 'Orange Wine.' 1900. Family mince pies 1890 superseded Historic Chutney from 1780 adopted by Uncle John.

Sadly we don't have a clan haggis, or a tribal cava cava.

Some obvious words are ancient, primaeval, primordial, mediaeval, paleolithic, with phrases available such as from the stone age, since Adam was a boy, from the Dark Ages, among others.

Consider,

immemorial

: very old or ancient : from a time so long ago that it cannot be remembered M-W

: originating in the distant past; very old : an immemorial custom OED

[existing] since time immemorial

: since a very long time ago. Literally, since time before recorded history. McGraw-Hill Dictionary of American Idioms and Phrasal Verbs

prehistoric is a possible adjective that fits your question, but it really depends on context M-W:

: very old or outdated