Of course it's a moving target, but I don't see that as a problem because life constantly moves...if we all chased in our 40's what made us happy when we were 5 years old it'd be a pretty fucked up situation. So what we see as happiness is a moving target...the problem is many have an unclear view of what that is I think.
Also yep, constantly seeking more and more (consumerism for instance) won't lead to true happiness.