Talk of half-lives usually implies a random event that causes decay. The population of the decaying substance then follows an exponential decline that may be mathematically described by...
This leads to being able to determine the half life as....
While all molecules will eventually be changed (if only in the final death-throws of the universe), I think it could cause confusion to say that 'all molecules have a half-life' as it would imply a mathematical pattern of decay that does not usually occur (as other reactions are likely to occur first). So I might say that the statement 'all molecules have a half-life' is strictly true, but rather unhelpful at the same time.
