Time, to many scientists appears to be relativistic, meaning the rate at which time flows is different now than in earlier stages of existence.
I like Sam2’s explanation (2nd paragraph). HKB”H can create that which appears to us as being very old, but in reality still exists within the 6000 (or 12 000) year timeframe.
There are rare cases in medicine/biology of humans having accelerated lives, meaning a 20 year old could appear to be in his mid 50’s, etc…
And did we not already read the Haggadah that says Rebbe Elazar Ben Azarya (18 at the time) had a 70 year old beard/face?
The Mekubal Rav Yitzhak D’Min Acco (a student of the Ramba”n), 13th century resolved the time differential and came up with a computation in the billion year magnitude.