Many people fund 529 plans to support a child's post-secondary education or training.
Please consult your attorney before acting on anything here, but our understanding is that this is a problem for special needs children.
The trouble is that if a child has 529 assets in their name when they turn 18, the assets over $3000 mean they will not qualify for disability associated medical assistance until the assets are depleted.
Instead some advisors suggest the creation of a supplemental needs trust (not a special needs trust). These can shelter funds for a special needs adult so that disability and medical assistance support is not impacted.
The supplemental needs trust can also fund education. For example, one could purchase tax free municipal bonds in the trust. The fund trustee, usually a parent or family member, can then use the funds to pay for education or training.
Some employers will also allow certain high income employees to defer compensation. It may be possible to then shift the deferred compensation to the fund. This definitely requires attorney review!
If you already have a 529 plan assigned to a special needs child, it can be shifted to another family member before the child turns 18.
To repeat -- you should not act on anything in this post without consulting a very experienced financial consultant or attorney. For example, Wikipedia has conflicting statements on the topic.