If you look at the raw numbers, they claim that there are nearly a million users each at ages 5 and 6 -- and a far-more-believable 800 at age 7. The numbers for ages 5 and 6 must be some kind of glitch; I'd guess that they are buckets that are used for people who don't actually give their ages, but that's just a guess.

My guess would be, a year-of-birth field with nothing entered is filled with one or more 0s. Later on, the statistical program comes along and reads "00" as 2000. Subtract date of birth from current date, gives ages of 5 and 6 (depending on month).