Vedas are Anadi, Ananta and Apaurusheya - eternal, infinite and uncreated.
That means they exist forever - before, after and during - Srushti, Sthithi and Pralaya.
The entire modern/scientific thought is entrenched in the notion of everything having a beginning and an end.
Unless you grasp the concept of infinite eternity, you can't fully understand Atma or Paramatma or most Sanatan Dharma spiritual viewpoints.
That being said, if you want to calculate the age of 'current' cycle of creation, and choose that as 'starting date' of Vedas, you can look at either when the current Kalpa began, or when current Brahma's lifespan began.
There are several answers on this site that explain Yuga timeline. I can post later.
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