Many physical things become difficult.
Most of the other things statistically work out when you start as a kid.
What are some sports, creative activities which don't have to be your full time thing but still you could spend years to understand and excel it?
Running comes to the mind, what would be other examples?
eg:
> Albert Ernest Clifford Young OAM (8 February 1922 – 2 November 2003) was an Australian potato farmer from Beech Forest, Victoria.
> He was best known for his unexpected win of the inaugural Sydney to Melbourne Ultramarathon in 1983 at 61 years of age
NB: this is literally an instance in which YMMV . . .
Blacksmithing (either sword/knife or artistic)
Photography, not physical, and the basics can be learnt with a simple course.
Anything to do with machining/lathes metalwork, the only barrier is cost and time.
Music, Its perfectly possible to learn an instrument and be good, world class is a difficult term though.
General knowledge, paul Sinha was a doctor turned comedian, then pivoted to "quizzer"
Story teller, life experience can be an excellent spring board to creating good narratives
Cuneiform scholar: https://www.khanacademy.org/humanities/ancient-art-civilizat... There are tonnes of tablets yet to be deciphered
And yet, in practice, almost all chess grandmasters have started young. I don't know of any grandmaster who started playing chess seriously past 30 (and chess.com didn't have any concrete examples that I saw: https://www.chess.com/forum/view/general/gmsims-who-started-...)
Physical sports are obviously out, as the world level is dominated by people under 30 for all of them.
I'll also comment further on two things you wrote in your original question:
1. "compete at world level" and 2. "don't have to be your full time thing"
In practice, the majority of things that have world competitions will also have people who will devote themselves, body and soul, to it, so if you are intent at not spending all your time on this, not making it your passion, then you should not reasonably expect to "compete at world level".
I think you might do better by classifying "compete at world level" as being very broad, like considering writing a "new york times best seller" to be competing at writing at the world level (in which case, yes, some authors started after 30). If you broaden it to simply "be able to have fun", I think you'll have a far better time of it, and at that point almost any physical activity now becomes something you can start past 30.
Now, you're not going to compete in SC2 with people who practice 10-20 hours a week unless you also practice 10-20 hours a week; but you can certainly get pretty good and have a lot of fun. (Maybe neurological limitations from being older would prevent you from being top-tier even if you did practice 10-20 hours a week; but I'm inclined to believe it's more that older people are less likely to practice 10-20 hours a week for whatever reason.)
To tell the truth, if you have not picked some skill which you are going to be a world level of, in your 10s, then there is no hope you will do it in 30s. Your question is awesome to see how little of people (even such brilliant ones as HN crowd) can see this obvious thing.
Reading comments is so funny, everybody has their confirmation bias. For example, from my PoV the easiest stuff you can do is metal cutting/wood turning. Just buy a lathe, solve that safety issues for your eyes and fingers (this is the hardest part, no jokes), read few tens of books on metals, than work few years with a more and more competitive master - volia, you are a world-class metal cutter... Then I remembered, that not every child use to have the skills I have.
If you have not hammered a few thousand nails in your childhood than you need to hammer tens of thousand in adult age just to achieve my level of hammering. Obviously to hammer a thousand nails in childhood is in extreme way simpler than to hammer ten thousand nails in 30s. Another example of sad trend of being older is just a reading books. If you don't use to read books in your childhood, there is no way you will start it in your 30s, having such issues as: bad eyes, hard work, big house to take care of, demanding spouse, growing kids, and of course - lost ability to enjoy reading.
OK, while writing this comment, I realized what is the best answer to your question. This is aging. Any 30-yo (who has not done too danger things for longevity like drinking too much of alcohol or having a contact with too danger chemicals) can pretend to compete at a world level about who will have a longer life. I am really convinced that a significant amount of today's 30-years-olds from HN crowd will live to 100 yo, which is a world-level competition from all points of view.
When it comes to sports I think it needs to be something with lower physical bar such as table tennis, archery, shooting. Unlikely you can compete on world level but at least locally.
For other things I think certain computer games might do but you will be competing with kids that play 12 hours every day so again you can only compete on small scale and in my experience online competitive games attract the most toxic people. You can try Geoguessr which I think is your best option.
Then you have the more arty skills such as stand-up comedy, music production, cooking all can be competitive if you are into any of these and your age shouldn't matter much in fact it might help.
It's a weird thing, it looks physical and it seems like you need fast reflexes to do it on the highest level but it's not true. The technique can be picked up in ~2 years with 2 hours of daily practice. What you need and what is hard to get is a calm mind and good decision making which could take a long time to get right, but some people are just born with that. You can easily start in your 30s and be at the highest level, or even be a world champion ( I've seen a few people starting in their 20s and get to that ).
Bjj tournaments have age brackets. Check the IBJJF worlds masters. If you win your bracket you do technically get the titles of world champ.
Before you start running, learn warm up. And take it seriously. It's the most important part of running.
Your knees in your 40ies will thank you for it. (My knees don't.)
In addition, you meet new people by discussing solutions to problems (routes on the wall), which is a major benefit for people in their 30s.
At least for rapier and sabre, many people start late in the 30s and can compete pretty well internationally (see https://hemaratings.com/)
Same with sword and buckler, Though I believe longsword and armored fighting needs more athleticism than other arts.
Feel free to see if you have a club nearby.
So you could learn to sail in your 30s, spend 10 years getting good (need plenty of time on the water and coaching) and then enter the masters.
Sailing is a sport where you need lots of time on the water, coaching and some luck.
But a lot of people drop out of it in the 20s and 30s due to families and work.
You learn something new about yourself every day. And you excel again and again only to learn that there is more to learn.
Changes how you think, live and breath.
Plus you look awesome after some time.
Although I'm of the opinion they're more fun as an activity than to do competitively.. but for many that's the fun part.
That said - you will never have articles written about you, be interviewed for X, etc. Dance is mostly for enjoyment and socializing. There’s almost no money in the industry as well as compared to running or other hobbies/sports. Most dances are funded by passion - not through capitalism.