There's readiness, and then there's readiness.
There's intellectual readiness for stuff like learning to read and do arithmetic. Then there's the emotional readiness for adult-directed goal-oriented learning work. Sometimes they both click at age 6, sometimes at age 5 or even 4, sometimes neither clicks until 8. But most often a child become intellectually ready for early academic learning at a younger age than he or she becomes emotionally ready for school-like structure.
That's why for years kindergarten was about play and exposure, not deskwork and achievement markers. That's why some of the highest-achieving school systems in the world, like those in Scandinavia, start academic schooling a year or two or three later than is typical in North America.
All of which is to say that your little guy is no doubt ready to learn but he may not yet be ready for the structure of a formal curriculum. You likely won't know unless you try it, but try it gently at first. Start with one or two strands and see how he's responding after a couple of weeks. If he's thriving, offer a bit more. If at any point he seems to be losing interest, and you feel like you're having to prod and cajole to keep him engaged, back off the formal stuff and move instead to a more organic, free-form, play-based approach to learning. Try again in 3 months. You may be surprised: a lot of maturity can take hold in 3 months, but a lot of learning often happens in the same space of time despite the absence of "schooling." I often find that when I set aside something because my kid doesn't seem ready for it, when we come back to it after a break, they've already learned it. How? Hard to say. When they're ready for the learning, they seem to absorb it with breathless efficiency just from the world around them.
My kids used no curriculum at all until the 2nd grade level, because during the time they were blowing through KG and 1st grade level learning their intellectual readiness far exceeded their emotional readiness for other-directed structure. Even at a 2nd grade level we only used a small bit of curriculum, gradually introducing more over the next 3 or 4 years as they seemed eager for it. So if you discover that your little guy is only ready for a little "schooling" next year that doesn't necessarily mean he won't knock back the entire set of KG milestones anyway.
As to whether we "officially started KG before 5," I really can't say. In one way, yes. My kids were all at a solid 1st or 2nd grade level by age 5.5, so they mastered the typical KG learning outcomes at age 3/4/5. And where we live my late-fall-birthday kids were only 4.75 the year they were officially registered as KG'ers with the government. But we didn't do any KG curriculum, not at age 4.75, not at age 5, not ever. So in another way, no, we didn't officially start KG before age 5.
If you are the sort who wishes to use a curriculum at this level, start gently. Use your child's enjoyment and engagement as a guide as to whether to do more or less of the curriculum. Play it by ear. He'll tell you how much is right for him if you observe carefully for his giggles, his sighs, his focus or his wandering attention. And enjoy! Truly, enjoy!
Miranda