Math and advertising

Over at the informative All About Advertising Law blog from Venable, the authors ask "what does a lifetime supply really mean?"  They describe one calculation method that, as described, seems obviously wrong to me: "For the Gumball.com promotion, the company took the average life expectancy of a U.S. citizen, subtracted it by 18 (the minimum age of entry) and multiplied the difference by 365, representing one gumball per day."  Though the US is a low infant-mortality nation and the final number doesn't differ much, the proper calculation is life expectancy at age 18, which isn't the same as life expectancy at age 0 - 18.  Science is the lawyers' friend!