I got the guardianship done this year, DS is age 21, Downs and Autism. Not really verbal, and does not really understand much about reality..... But I am not 100% sure I HAD to do it, but I guess it is better to be done. MORE important, but still not done, is setting up a special needs trust and fixing our wills. Right now, DH and I have lots of life insurance, and if something happened, he would inherit 1/4 of it, and lose his SSI and health insurance until he spent it all. Then he would be able to go back on SSI and regain it, eventually, but would have no backup trust $$ for his needs. Scary.
It cost me about $4000 total, for the guardianship; here in NJ it is a big complicated thing. Loads of reports, paperwork, 2 doctors had to sign off (the psychiatrist charged $700, the other was filled out "free" from his internal medicine DR).
NJ used to have a program to do it, but the waiting list is so long, he would be in his 30s before we reached the top. They also allowed us to request funding to pay us back for some of the cost if we used a private attorney. However this is a program they approved in Trenton (state capital) but they have not funded it in years. If they ever DO, we may get some of the $$ back. We are on that list. But I am 100% sure they will NEVER fund it. (state is broke, and they like to pass feel good legislation, hold a press conference announcing it, but they just don't fund it! UGH, I hate NJ sometimes!!)