I do wish I had finished a year earlier, though. It would have meant cheaper college and would have also meant starting a career sooner, which for me is above $100k/yr. It would have also increased the chance of my mother still being alive since she died right after I was hired and I think if she didn't have to work until the end because I'd have been paying her bills she'd still be around.
I'd suggest going with what your kids want. If they want to learn, go for it. If they're perfectly happy with pre-school/daycare, or have lots of friends in that age group, maybe reconsider. They'll make new friends regardless of the grade, though.
My son was born in early July and my wife and I are trying to see if we should wait until he's a full 5 and be a year older than everyone or go when he's basically 4.
The extra year in a job to me isn't trivial. It's a 50k + extra year of experience decision. I left high school a year early and even though I'm the same age as a lot of people around me, I have 20k extra in my 401(k), I'm in a management position and I have graduate school finished.
The start advantage is fading as I get older though.
It probably still depends on the child. Some can probably handle it better than others. Personally, I'll have to wait and see to see what the little guy is like and what he can handle.
The right answer is evaluate what's right for the child. Different kids have different maturity levels. My daughter started K at age 4 and was still well above average and bored by 5th grade. The productive thing that school districts can do is roll out pre-K so parents don't have to make a costly daycare choice.