In performing the calculation of the universe's age it is acceptable to assume that general relativity and other well established physical theories are correct and that the universe is effectively homogeneous and isotropic at large scales (unless there is significant evidence to the contrary). Uncertainty due to effects of quantum gravity can be assumed to be insignificant.
It is not acceptable to assume that the cosmological constant is zero or make any other assumptions about cosmological parameters, such as matter density, without taking into account uncertainty in their measured value or limits. It is not acceptable to make unsupportable extrapolations or to rely on fine tuning arguments. It is not acceptable to make assumptions based on theories which are not well established such as the cosmological inflation theory (unless they become well established by time the claim is judged).
If the theory that the big bang never happened becomes widely accepted by professional astronomers and the universe is thereby shown to be infinitely old then YES coupons pay. However, in a theory of a cyclic universe in which each cycle begins with a big bang, or a universe in which the big bang was a local incident in an older universe the age of the universe will still be taken as its age since the big bang. If it is shown that the universe is exactly cyclic with no big bang and the cycle time is less than 13 Gy or if any other accepted theory makes the meaning of the claim ambiguous then pay-out is split 50/50 between YES and NO.
Standard ambiguity clause: If the wording is found to be ambiguous I will judge on the basis of the obvious intent. If the intent is ambiguous I will judge on the basis of precise wording. If both are ambiguous or if both are clear but conflict, I will look for a solution that causes the least damage to FX as a market and game.