* Evolution, not a sudden creation: Languages evolve over time. They don't have a sudden "birth." Welsh is part of the Brittonic branch of the Insular Celtic language family, which itself evolved from Proto-Celtic, a language spoken in Europe thousands of years ago.
* Historical evidence: We have written evidence of Welsh from the 6th century onwards, with the earliest known inscription on the "Dolau Cothi Stone." However, this doesn't mean the language began then; it simply means we have written records from that point.
* Oral tradition: Languages are spoken long before they are written down. Welsh has a long and rich oral tradition, which predates any written records.
Therefore, it's safe to say that Welsh has been spoken in Wales for many centuries, possibly even millennia, but a specific date for its "beginning" cannot be determined.