Andy:
Um, that is a bit fuzzy. If you are going to take the average balance by splitting the beginning and ending sums, then you must also take average return over the same period: 0% to start plus 5% to end = 2.5% over the period--you're essentially mixing units. It is absolutely important to recall the return is not based on an average balance, but the total at the end of the period. I could start with 1301 dollars IN and take OUT all but 1 dollar at the terminus and still have an average of 650 dollars; with a return of $.05, by your logic, my annual return would be .007% x 2 = .014%, though in actuality, it was still 5%. As you know, annual rates of return are not based on periodic averages, but differences between start and end periods divided by return--the units remain constant regardless of how calculated.
When the Feds claim a 4.00% average annual GDP growth they multiple a quarterly return times the number of quarter, i.e., a 1QFY10 GDP growth of 1.25% would yield a 5% GDP growth rate. They do not say the 0.5% over hald the period is worth 8% annually. The concept is purely additive...
PS: You're confounding knowledge of economics/accounting with years of service. Simply having money to retire is not solely a function of knowledge--I'd be willing to bet age more than anything. If we controlled for age, you may find some of us arguing here and you may have equivalent net worths...