I think my experience sounds similar to Captainbird's.
There's 7 years between mine.
And it's been perfect. They're now 27 and 20.
There was enough of an age gap for me to never feel overwhelmed by parenting. I read threads where women talk about the pressures of 2 under 2 or 3 under 5 and how draining and exhausting it can be. I didn't have any of that.
There's never been any sibling rivalry because of the age gap.
They rarely bickered or fell out - not until teens really - because he was always old enough to understand that she was just very little when she was annoying him rather than also being very little himself.
He used to have sleepovers for her in his room when she was little and she loved spending time with her bg brother.
He's always been fiercely protective of her and she has always trusted his advice.
As adults, they've been for weekends away together and she got to do very grown up things like being taken out for dinner by him and his girlfriend and his girlfriend's similarly aged younger sister, which was far cooler than being taken out for dinner by her mum! 😉
He's helped her with homework, friendship issues, boyfriend problems... and now she's old enough to do the same in return for him - well, not the homework but definitely girlfriend issues!
As adults, they are incredibly close. She's away at university and I know they're in contact nearly every day. If she messages me to tell me some good news, I know he already knows.
Of course, they have disagreements but there have never been any major problems or fights, extended arguing, screaming etc and I put a lot of that down to the age difference.
In short, I think the age gap is less important than the combination of their personalities and how you help them navigate their relationship as siblings.