The nice thing about forums like SI is that there are literally thousands of folks who are either going through the exact same thing you are or have been through it already. You will get many different opinions on issues such as this. In the end, you know your situation best and you will make the best decision for you.
Based on my own experiences, I firmly believe that everyone, no matter what age, can better deal with major hardships like divorce if they are given the truth from the beginning. Problems have a tendency to arise down the line if we, as parents, withhold the truth or whitewash it in hopes of protecting our children from some of the ugly realities of life.
(I also feel that if you don't tell your children the truth, you are, in effect, protecting their father and his mistress from some of the consequences of their actions such as the inevitable uncomfortable questions/comments.)
My boys were young - 10 & 12 I think - when I had to have The Talk with them, and they did fine. Just explain things in an age-appropriate way without editorializing and be ready for many, many questions...and hugs. If they're not already going, I'd suggest a good family therapist, just to keep things on track during some of the upcoming bumps.
I'm a few years out from that and we've all not only survived - but thrived! Compared to being married and always waiting for the other shoe to drop, being divorced is heaven.
Keep posting, we're here for you. Remember, you and your kids WILL be okay.