My very first diagnosis was Autism when i was 4yr. my family fought it though... leading through a massive list of misdiagnosis' during my entire school life. it wasn't until the past year that they finally started accepting that the doctors were right.. however they dont appolize...
I think with being on the spectrum, there area lot of things that happen, which can be misunderstood, and seen as something different. Leading to an almost impossible hill for the child to climb and to accomplish. Because certain ways to do things, aren't the ways that help. What others see as a tantrum, and can and should be controlled by the kid, or its a behavior problem... is really a meltdown, and by getting mad and adding more stimilus to the kid will only cause things to get worse.. such as disiplining the child while having a meltdown. spanking a kid for what appears to be a tantrum, does nothing more than torment the autisitic child, as it furthers the overbearing stimulis, and another extreme. (that's something that happened quite often for me).
But by having a reason, which gives more understanding, it opens up that area of "well maybe this is managable", because often it's an rough path for both the child and the parents if they aren't able to see what's really happening. such as, a tantrum is associated to a "it's not fair" anger and hurt feeling, and when asked why they are being the way they are, they can clearly state what it is that they want, loud and clear. and tantrums are a behavior issue..
While meltdowns can come on "out of the blue", and the cause might not be able to be determined. But it is uncontrolable, it happens, the sense get totally overwhelmed in the brain, which causes a meltdown. at these times, a child might not be able to explain why, or whats wrong, or even after the meltdown and they are calmed back down, usually they are as confused over it, as you are.
a ton of people thought i would be lieing when i was telling the true about things.. because of my body language, and the fact that i wasn't crying when revealing something tramatic, and sometimes i would laugh inapproapriately. And i wasn't able to understand or figure out, why people wouldn't believe me. in daycare one day i even had a worker chase me down as i was fleeing the overwhelming stimiuls, then yell at me, and because her kid told her i did something that i didn't, she was furious, and told me she knew i was lieing.. after she went on and on, it finally came to a stop when she asked me if i knew how she knew i was lieing, and then told me because i wouldn't look at her.... that left me even more confused, and i withdrew because no one had ever pointed out that i didn't make eye contact... and i didn't know eye contact was one way to be believed. i also got into more trouble for "lieing" because i was trying to tell her how i wasn't lieing. After that they placed me with the 2 year olds group. I was 11-ish.. however, i didn't see that age gap..

I think if my parents would of agreed with it, and didn't pull me up and move everytime someone came to the conclussion of autism about me, then i would of had a lot more progress than where i am currently at.
Such things have an affect on both the mental well-being of the child, and also the parents, or caregivers.
When i was younger, i think Autism was still viewed a lot as like a terminal illness... and rendered the parents into a helpless type of feeling for their children... which no parent wants to feel like they are there on the side lines, and are completely helpless in helping their child.

I think having a diagnosis, adds to the aid and help, along with a greater understanding, in which can then lead to finding ways that work.

Something one of my casemangers said (which is very accurate for me), you just have to know the right questions to ask.
Kind of like:
person 1: do you like coffee?
person 2: kind of
1: what kind do you like?
2: I dont know.
1: do you like decaf coffee?
2: no.
1: do you like hot chocolate coffee?
2: no.
1: well what type of coffee do you like?
2: I dont know..
1: what about pumpkin spice coffee?
2: no.
1: do you like cinnamon coffee?
2: yes.
1: do you want a cinnamon coffee?
2: YES!
Which can be really challenging. but the answer does end up being found out, where otherwise, it might remain unknown. It's kind of a langauge difficulty. And differnt forms of communication might make this a bit easier. Anywhere from highlighting sentences that are true for them, to other forms of non speaking ways, can help make this kind of thing easier. But when asked a open ended question, or a "fill in the blank" type of question, they may be stuck and unable to get any of it out into explanations or words.(in my own experience).

For older people, a person can try to figure out if having the diagnosis will help them any, or if they feel like it would harm them. Eventually leading to them either reaching out of the diagnosis, or leaving it behind.