The broader point is that being a couple days or months younger or older than eighteen years should not result in a drastically different outcomes to begin with.
The problem is that anytime you create a division there are always some cases just on the border, and if the outcome difference is sized to the difference between the centers of the classes the jump from on size of the border to another gets large.
But on the other hand if the jump isn't large you end up saying that 13 year olds and 30 year olds should be treated the same, which is clearly just worse.
You can try smoothing out the jump at the edges by adding more cuts and doing less changes from one step to the next, but I don't think you gain much in terms of recruitment of youths compared to just pushing the breakpoint fore lenient sentences to larger ages than 18.