In your experience.
I have a toddler, 18 months old. On Victoria Day, I bought a bunch of fireworks, hoping he’d enjoy them (held firmly at a safe distance) - but he was obviously too young; the loud bangs and bright lights just upset him. On the other hand, his three year old cousin was delighted.
Obviously, at some time between 18 months and 3 years, kids go from dislikeing fireworks to loving them (no doubt with individual variations). What was your experience?
My youngest is 15 months and there is no way he’s going to any sort of fireworks display this year. My oldest is almost four and he’d freak, too. He is very sensitive to loud noises. My brother hated fireworks growing up and never wanted to go. He’s 43 now and still won’t be around them.
I don’t know the answer to your question, but I suspect it’s so highly variable that there really isn’t an answer. Regardless, I don’t think I’d even try with anyone under four and that would be dependent on personality.
I distinctly remember going to see fireworks when my son was a year old, that puts his sister at 2 1/2 years old. (Yeah, it was a busy time!)
Anyway, the one year old had no problems, but the 2 1/2 year old did: too loud and scary. The one year old was too young to be scared. The following year I think they were both OK with it.
So, as all things are with kids, it depends on the individual. Probably somewhere around 2 they’re old enough to realize it’s scary, and sometime between 3 and 4 they realize Mommy and Daddy probably wouldn’t let them get hurt by the loud fiery explosions. (Unless you’ve given him reason to believe you’d hurt him with loud, fiery explosions, of course.)
Took me about 18 years, personally. Before that they were just really loud, unpredictable firebursts that may or may not be able to kill me.
Could be that the kid will either love them or hate them. I guess you’ll know in another 18 months 
As I recall, my kids were maybe 3 to 5 or so before they enjoyed them.
FWIW, last year my youngest was 22 months, and when I set off the first one, she freaked, jumped into her mother’s lap, and loved the rest of the show.
Big variation. I’d say three is generally still iffy - and that most kids like them by five or six or seven. But some remain scared of the loud noises for a lot longer and some are never frightened of them at all. And some does depend on the context - Mommy’s lap may be way safer than going with Daddy. My kids were spooked when we took them to fireworks as preschoolers, but reportedly loved it when they went with Grandma and Grandpa. Crowds may be as scary as the noise and compound the problem - or they might be comforting. Some kids handle fireworks on the Fourth fine, then get to WDW or Disneyland and see the fireworks and dragon during Fantasmic and freak.
Kids aren’t predictable.
I think most kids 3+ like the ones that aren’t loud and bangy – sparklers and roman candles that just make pretty lights and a sort of a fizz/hiss.
The bangy ones are a whole different story. I still don’t like the “flash & bang” ones you see a lot – its just a bright flash and a bang, no pretty sparkly streamers or colors or anything. They kinda scare me, plus I feel cheated, all at the same time.