When your disabled child turns 18, you lose your right to Parental Leave.
Once you have worked for your employer for a year, you are entitled to take 18 weeks' unpaid parental leave for each disabled (gets DLA) child, to take before your child turns 18, and 13 weeks altogether for each non-disabled child, to take before your child turns five. Check your contract or with human resources, as your employer may make more generous provision. If your contract doesn't specify, you can usually only take four weeks per child during any one year unless your contract gives you longer, but you can always ask your employer to be more flexible. You are also supposed to give 21 days' notice, but again, you should ask your employer because they may be able to be more flexible.
If you need extra time because of your caring commitments, you can of course request some unpaid leave, but as a carer of an adult you do not have the right to it. You are still entitled to a short time off if an emergency arises. An emergency includes not only the sickness of a relative, but also the breakdown of normal care arrangements.