My son noticed that his oldest son turned 18 but only about 1/5 of the support is coming off, the youngest son is allocated 3/5 of the child support, which means tens of thousands more dollars to the ex, than it would if the support were the same for each child. Is this normal in California? Can he request a modification to make them even?

It is not really that, you pay so much for one child and then so much additional for the other child. So no it does not drop by 1/2 if you have one child drop off.

Your best bet is to go online and use a dissomaster and calculate what the child support should be. Since there is a different spread now then before it may or may not make a difference.

Thanks, Fr_Chuck, but there are three children, and father thought it was 33.3% for each, but now that oldest is about to be 18+graduated, surprise, only 20% comes off! Likewise only 20% for the second. 60% of the amount is allocated to the youngest son! The articles / Nolo Press I've read are clear that parents can make whatever agreement, and judge most likely explained the guidelines, so I'm figuring he was fatigued from the processes, and wasn't reading everything and she knew it. Since it's still several years until the youngest is 18, the difference is dramatic, in the 10s of thousands of dollars. The only other obvious guidelines I found were if the oldest is near 18 at the time of settlement sometimes they recommend a slightly higher portion for the oldest, but I'd never heard of doing it for the youngest, obviously crafty atty and ex who had an idea they would get away with it. Thanks, califdadof3, looks like Dissomaster costs a few hundred bucks so I'll leave that to him.

Here's the calculator - Calculate Child Support

Are you saying that because of a "young, crafty Attorney" your son is paying MORE than the Court calculator would indicate?

Normally you pay what the state orders, almost never do you pay less, sometimes you pay more. But most just pay according to a very specific schedule. And no it is not 33 percent per child,

OK I see the long list of considerations, which the calculator starts as being the same for each child, with lots of considerations that can be different for each. I was quite surprised that in all the 31 selections in Advanced (timeshare selection) not one was alternate weeks (one for each parent)! So I punched in 15 or 16 days per month on one, just entered 50.0% on the others (though really he has them more than she does). Indeed it came back with a total, broken down to about 20% for the first born, 30% for the second, and 50% for the third, wild I would never have guessed that!

@Judy, no, not a young crafty atty. I was speculating that the ex's (older crafty) attorney, had skewed this schedule to triple the costs of the youngest child, but now I see this calculator comes out similarly, so I'll look for why that is. So, thanks, Judy for the calculator, and everyone else for helping too.
For average American income, the skew is small, but for large incomes it definitely calculates a much larger amount for the youngest child. So the answer to the original question is "Yes, the State recommended unevenness becomes greater to the degree total income is greater"