You have a two year statute of limitations in every state except NC and LA (which are both one year) to file a Due Process Complaint for a school's failure to provide your child with a FAPE. They don't have to be enrolled currently for this to happen and this includes the two years after they graduate or age out of public education.
The two year statute is constantly rolling as time passes so keep that in mind. But, let's say that your child has been in a public school for years and it's just not working and your child isn't making progress and you decide to move your child to a public Charter School. You would be able to file a due process complaint against the old school for the prior two school years when your child didn't make progress and seek remedial/compensatory services to offset the services that the Charter School is required to provide through the IEP. We frequently help families do this and what we typically negotiate is a block of funds that the old school district sets aside for the parent to secure private tutoring or therapeutic services or a transfer or reimbursement of funds from the old school to the new school to offset additional costs and supports that the new school has agreed to provide over and above what is required in the IEP (knowing that the old school is funding those solutions/supports).