Apologies in advance for any ignorance, I'm a non-linguist hoping to better understand the methods in the field (if any) to answer a question I have. In particular, I want to know when a word first appeared in a language tree.

For instance, if we take the word "iron," I see that the earliest version in the Wiktionary (<https://en.wiktionary.org/wiki/iron#Etymology>) is from Proto-Celtic, derived from the PIE for "blood." So, perhaps I could naively date the word to around 1000 BC, the rough age of Proto-Celtic (<https://en.wikipedia.org/wiki/Proto-Celtic_language>).

My question is what the most proper and formal way to do this is for a large number of words and languages. Should I:

1. Be looking at etymological dictionaries / estimated origin dates for languages?
2. Looking at some computational model conducted at the word level?
3. Give up because we don't know enough to know these sorts of things? Of course, I don't need to know the word appeared on January 12th, 1427 BC. The question is, is there any method that improves beyond naive guesses?

Appreciate very much the help in this area--thanks a lot!