We max our HSA every year. We figure that by the time we are old, we will get sick. So it becomes a gamble on whether you think healthcare in the US will continue the way it is going (individuals shouldering more and more of the cost burden) or reverse course and get costs under control. I am voting for #1. So we figure we might as well put as much as we can in the HSA now, and use it when we are old. I believe that once you are a certain age you can also use it for any retirement expenses, not just medical.
That said, so far we have used the whole HSA fund. With an HSA and 2 kids + 2 adults we had 6K in OOP medical costs last year. We are dealing with some genetic issues that we didn't know about and couldn't predict at age 25 ;) Your health at 25 isn't a great predictor of your health in 5, 10, 20 years even if you take out lifestyle-affected illnesses. It sucks but at least we could pay it pre-tax.