They have a age cutoff at 18. Do you know why diagnostic tools like this that are so non-invasive and just a regular blood sample collection is not available for peds? I complained a bit on Twitter this morning.
I don't know specifically for this case, but it could be something as simple as when they went to the freezer to pull out the hundreds or thousands of stored serum specimens to develop and/or validate the tool, they mostly had blood from adult Crohn's patients - thus not enough data to to say whether it would work for children too. And pediatric Crohn's is often thought to be somewhat different, perhaps in some cases such as VEO, a completely separate disease. Thus, pediatric Crohn's will likely require its own tool that has not been perfected yet.
It is generally true in developing drugs, medical devices, or blood tests that you start with adults and work your way down to the kids. Research using children can be more difficult and more expensive than with adults. To begin with, children cannot give informed consent for themselves to participate in medical research. Their parents must provide it for them. And many parents balk at the thought of their children being used as "guinea pigs," and refuse to provide consent. Which of course slows enrollment in the trials. When you see an age cut-off of 18 that usually means that patients under age 18 were not included in the key clinical trials, because 18 is the age at which you can begin to give informed consent for yourself.
They may be working on version for kids or extending this tool if they can show that it works for kids the same way. I don't know whether they are. You see this stepwise approach taken all the time - develop the first version for adults and later bring out the pediatric version once it has proven safe and effective in adults. We saw this in the roll-out of the COVID vaccines: adults first, followed by older kids, followed by younger kids, followed by very young children.
Researchers, parents, and the FDA are all more cautious when it comes to children, so everything just takes longer and costs more. And if the market for pediatric patients is not big enough to be worth it for the company to spend all that extra time and money, the kids may never get the new thing at all.