Not necessarily a wet dream. Orgasming can happen before one is physically able to ejaculate. 

Erotic feelings develop before puberty. I distinctly recall being turned on by a 2nd grade classmate.  

My first ejaculation happened when I (M) was 11, but I experienced orgasms and discovered masturbation at least a year before that.

I remember having one or two more wet dreams when I became a teenager, but they were actually wet (spermy, unlike the first time when I was a child), so I thought this was a different from a wet dream

It’s called a wet dream. Though that seems very young to have one!   It’s perfectly normal, nothing to worry about. Some ppl have them, others don’t.

What distinction is there between "responding to sexual stimuli" and "sexual attraction"?

Isn't mailing off for sexy pictures a "sexual behavior"? Isn't ogling boys in sexy bathing suits and hounding Mom to buy me one a "sexual behavior"? If not, why not?

I'm not understanding these distinctions.

>It's important to distinguish between brain response and body response.

Can we actually distinguish between them (in this context)?   How?

It's important to distinguish between brain response and body response. Body can respond to sexual stimuli at any age, it's a reflex we have from birth. Sexual attraction, sexual behavior normally forms at the start of puberty.

>Masturbation and orgasm before puberty are completely normal.

Can verify. I (M) discovered masturbation and was dry orgasming well before puberty.

>A lot of babies under age of 2 masturbate and orgasm, most of the time parents either simply don't understand what's happening or punish them so they stop. 

Indeed. My parents punished my childhood erotic play.  I distinctly recall several instances between 4 and 6yo (possibly younger, as you said, but i wouldn't remember those).  

Punishment didn't stop the feelings of course. It just suppressed them, with consequent physical and psychological traumas that I carried for decades.

At age 7, I began lying to my mom (and everyone else)  about those feelings... she'd made them unsafe.

>What changes during puberty is our brain begins to react to sexual stimuli.

My brain reacted to sexual - or at least erotic - stimuli long before puberty. My earliest memories include such responses,  from the age of 4 onward. 
- At 7yo I mailed off for a

We have the ability to orgasm already in the womb. All the wiring is there from the get go. What changes during puberty is our brain begins to react to sexual stimuli. A lot of babies under age of 2 masturbate and orgasm, most of the time parents either simply don't understand what's happening or punish them so they stop. Masturbation and orgasm before puberty are completely normal.

Why did I get an orgasm out of nowhere?