@OP, as per pp's advice, you can make a UC claim for your 19 year old DC in his own right.
The DWP may make you apply for a separate appointeeship. Yes, I know it's crazy, given that the DWP run PIP and the DWP run UC, but that's what they did to my DP. He ended up with two identical pieces of paper, with the same reference code in the corner, just with different dates on them. Go figure.
We ended up effectively rewriting the PIP2 form onto the separate UC50 form, enclosing copies of all the original supporting paperwork and reports, in order for his DS to be placed in the UC 'support group' (no requirement to work or look for work), as the UC dept don't and won't take into account any PIP award, even if the person is on both enhanced elements.
You then have to ask UC to apply any relevant disability premium, as this may not be done otherwise.
It is a system not fit for purpose; but I hope our experience might help others 