There are several quick, oft-cited model calculations used for dynamic asset allocation of a portfolio of stocks and bonds by age, moving more into bonds as time passes because they’re safer. For the sake of clarity and consistency of discussion, we’re going to assume a retirement age of 60.
The first and simplest adage is “age in bonds.” A 40-year-old would have 40% in bonds. This may indeed be fitting for an investor with a low tolerance for risk, but is too conservative in my opinion. In fact, this conventional wisdom that has been repeated ad nauseam goes against the recommended asset allocations of all the top target date fund managers. This calculation would mean a beginner investor at 20 years old would already have 20% bonds right out of the gate. This would very likely stifle early growth when accumulation is more important at the beginning of the investing horizon.
Another general rule of thumb is a more aggressive [age minus 20] for bond allocation. This calculation is much more in line with expert recommendations. This means the 40-year-old has 20% in bonds and the young investor has a portfolio of 100% stocks and no bonds at age 20. This also yields the stalwart 60/40 portfolio for a retiree at age 60.
A more optimal, albeit slightly more complex formula may be something like [(age-40)*2]. This means bonds don’t show up in the portfolio until age 40, allowing for maximum growth while early accumulation is more important, then accelerating the shift to prioritizing capital preservation nearing retirement age. This calculation seems to most closely follow the glide paths of the top target date funds.
Generally speaking, it could be said that these 3 formulas coincide with low, moderate, and high risk tolerances, respectively.
Asset Allocation by Age Chart
illustrates the 3 formulas above in the chart below: