The boys are 7 & 11. They have been traveling and camping their whole lives, so they did fine. We did 1000 miles (18 hours with stops) the first day. Total trip was 4500 miles.
3 years ago we did a 2-week Badlands/Yellowstone/Tetons/Rocky Mountains/Santa Fe trip with the whole family: wife, boys were 4 & 8, and my daughter was 2. They did fine, 2 year old camped, did long hauls, etc. like a pro.
We learned the key with little kids is to break up really long driving days with a long lunch stop where they can run around and get rid of some energy. Don't be afraid to push your kids--they can do more than you expect. Just know when they've hit a breaking point.
If you can make it happen, go as soon as you can. Too much can change in the future. I'm already thinking about future trips.