Yes, a very confusing way of wording the question. Am going to try and explain it - hopefully someone else will be able to do it better than me though.
Take your current age, subtract 10. Result = x
a) If x higher than 18 -> provide info for last 10 years
b) If x smaller than 18 -> provide info from your 18th birthday
c) If x = 18 then it depends on your actual birth date. I'd stick with a) just to be sure.
Another way of looking at it, again starting from your current age: count backwards, and what comes 1st? Your 18th birthday or counting back to 10 years? Whichever comes 1st as you go back are the years they are interested in.
Basically, if you are now older than 28, fill in for the last 10 years. If younger, fill in years since you were 18.