I've heard a few parents comment that they probably couldn't get guardianship of their adult child because the child was too high functioning. I've given this matter some considerable thought and from what I've seen I am rapidly coming to the conclusion that as long as you have two doctors willing to back you up, you most likely will be successful in getting guardianship even if your child is fairly high-functioning.
I expect to be in the same position myself in a few years with my oldest daughter. She is very verbal and very capable of making her wishes known but as far as I'm concerned (and as her neurologist agrees) the problem is that she doesn't and won't have the ability to manage her money, keep herself safe and live as independently as she would like.
And I don't think it can ever really hurt to try for guardianship if, in your opinion as the parent looking out for your child's best interests, it is is warranted. If you can get two doctors onside and as long as no one heads to court to object, I think chances are good you will be successful. And, if you're not, I don't see how you're really in any worse position than you were before.
And even if you have only one doctor who advises you that you are well-advised to seek guardianship (it seems like it often often happens that while a specialist might agree, your family doctor won't or it might be that your child only sees one doctor), that doesn't mean it's a hopeless proposition.
Sometimes a family doctor will 'change their tune', so to speak, if they are presented with a copy of the specialist physician's completed affidavit and will then agree to provide their own. Even though they initially refused.
Or, if not, you could always explain the situation to the one doctor you do have onside and ask if they could refer you to another doctor who might be willing to assist. Of course, that new doctor would have to meet with you and your child in order to do an assessment. But that likely wouldn't involve much (if any) more than one visit.
I was actually surprised by the response of our pediatric neurologist when I first broached the subject with him, asking if he would, when the time came, be willing to support me in an application for guardianship of my child. Yes, he would. But then he added, much to my surprise, that, in his view, guardianship applications are a colossal waste of time and money when it comes to young adults who very severely challenged because "no one is going to do anything without checking with Mom and Dad first". But that, in the case of young adults like my daughter, who will be so much more involved and active in the community, these are the ones for whom guardianship is essential to keep them safe.
And although I might well disagree with his assessment in the case of those who are very severely challenged, I most certainly do agree that we need something in place to protect our higher functioning children. And, at the moment, the only option on the table is guardianship.
I do think the one thing to be careful about with a high-functioning individual, though, is to how you present the guardianship application to them. To make sure you present it so they will see it as something positive ("your parents will always be here to help you with those things you find difficult") as opposed to something negative ("you're not able to do this, I won't allow you do do that") and thus be less likely to voice objections to the idea.