How old are the boys?

I'm watching the first season right now... I'm hearing some ages and guessing on some...

Francis: 16 (says in an episode)
Malcolm: 10 (says in an episode)
Reese: 11
Dewey: 8

In Season 6, Hal and Lois have their 20th anniversary. Since Francis was born the day of the wedding, he would be 20. Reese mentions he is almost 18. Earlier he had said he was a year older than Malcolm, but one episode had an 8-year-old Reese and 6-year-old Malcolm, which might make the difference a little more than a year, so Malcolm is 16 or 17. At least 16 as he's driving. In the first season, Dewey was supposed to be in first grade, which would make him about 6, so he would be 11.