As my sig says, I don't think I'm an autist, but I would say I've experienced meltdowns. But like you, only once I was, well, 17 exactly actually. That's the age I developed a very long, sometimes severe, depression. I also think that's when I developed my celiac disease too, but I'll never know that for sure.
Lots of mental illnesses strike young adults/old teenagers for the first time; depression, anxiety, schizophrenia, likely others I can't think of atm.
I do think I have some ASD traits, but I never really considered them as such until, oh, years into being depressed. They have become worst in the past decade. Desire to be antisocial, sounds and smells bothering me a lot.
I also seem to differ from what I read about most autist's experiences with meltdowns in that I never really thought of them as anxiety driven. But I think this might just be because I just don't do anxiety. I don't function when I'm anxious, like, at all. The vast majority of the time, anxiety is mixed in with something negative, so it's always a double-hit and I'd just shut down, go to sleep or just go to bed, shut out the world, often at the most inopportune times, and hope I feel better when I wake up. (I nearly always do/did.) I am very, very good at avoiding or dimming/diminishing any anxiety I might have, possibly because if I don't, I WILL shut down.
I also have improved a lot with this since being gluten free.
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Not autistic, I think
Prone to depression
Have celiac disease
Poor motivation