If someone is healthy for their age, should they forget the "age in bonds" (or similar) rules?
Suppose you are 40 but have the health of a 25 year old - you eat well, don't smoke, can run long distances, and are around the same weight or less than you were at 25. Should that play a factor in determining how much risk you take on?
That depends. If you're 40 and feel 25, does that mean you plan to retire at 80 instead of 65 and put off collecting social security until you're 85 instead of 70? And are sure you can pull it off? If so, then you can have a more aggressive asset allocation.
I think you'll find that most people around here, even people with average or sub-average health for their age, plan for retirement with the assumption that they may well need funds to support themselves well into their 90s. Even if the average life expectancy is around 78, it would be quite foolish for anyone not already diagnosed with a terminal illness to budget their funds around the assumption that they won't need money past that age. Perhaps excellent health gives some people better odds of making it to 95+, but just about everyone who is being responsible is already taking into account the chance that they might live that long, so I don't see how excellent health would make a difference unless someone thinks his/her excellent health means that instead of planning to live to 95+, s/he needs to plan to live to 110+. That strikes me as actuarially improbable, no matter how many miles one runs per day.
In other words, I think responsible asset allocation already presumes one will live a very long time, so I don't see how excellent health should affect it one way or the other.