Right. OP, there is no way that you can go back and change anything that has happened. So stop dwelling on the past and consider how you can go on from where you are now. It’s not true to say that you can’t succeed with a career change or a sport ambition once you are into your twenties, however it is a fact that it is harder to start from there but not impossible.
I changed my career in my mid-twenties to follow a career most people view as an impossibily difficult career to get in to. Sheer bloody-minded determination was the main driving factor that got me there. As a child I’d wanted to be a vet forever but was told when I was about 16 that I wasn’t clever enough. I had no idea what to do after that and spent the next 8 years with a vague sense of disappointment that I had not got where I wanted to. One nothing-job sent me on a training course abroad and the captain on the flight was a woman. I made the sudden decision that that was what I wanted to do. Everyone, not just my parents but EVERYONE told me that it was an even more ridiculous ambition than wanting to be a vet. 2 years later at the age of 26 I piloted my first passenger jet.
So if you want to change career, research it, work out how to do it, and then go after it like a dog with a bone. Keep at it, explore the funding opportunities, work out how you can acheive it, what the timescales are, and go for it.
But.... if it is sporting success you want, it is unlikely that you will be able to acheive sporting success and a career change at the same time. In your mid twenties though there is every reason to expect that with the same sheer hard work, dedication, commitment of time and all your resources, and - most importantly - the right sort of sport for someone to take up in their 20s there is no reason why you couldn’t represent your country. I know of numerous examples where people have taken up a sport late but with incredible success.
To start with, if you want to represent your country, you are right, you are past it for gymnastics and sprinting or pole vaulting. There is no reason though that you cannot take these up and thoroughly enjoy them. But if you want to win, to be recognised for your sporting successes, take up a target sport, or a skill sport or an endurance sport. All of these are sports well populated with older people who continue to improve as they get older. The article Villanellsproudmum linked to above set some of these out perfectly.
I have a friend, she went to a country fair and took part in a ‘have a go’ clay pigeon shooting competition. It was the first time in her life she had held a gun, but she shot well enough that she was given a wildcard to a newcomers competition and a voucher for some training. 5 years later, in her late 30s, she is lady’s champion in her country and is on her national team. She’s travelled the world and met her husband through clay pigeon shooting, but she works and trains bloody hard for it.
I took up a niche equestrian endurance/skill sport 2 years ago in my 40s and was 2nd nationally at it last year. I was beaten by a woman in her 60s, who was more skilled than me. I intend to go back and become national champion, and my ambition is by my 50s to be on the British team and represent my country. It’s not going to make me famous or my living, but I gain a great deal of satisfaction from doing it and doing it well. In between I take part in unusual races - I’ve been very briefly shown on TV (including The One Show and the BBC and ITV news) several times this year in races I took part in between other competitions.
I know people who’ve climbed Everest, who are world gliding champions, world aerobatic champions, champions in show horses at the Horse of the Year Show and Royal International Horse Show. People who sail around the world, kite surfing champions, someone who rode the 1000 mile Mongolian Derby, ultramarathon winners, Ironman triathlon winners, wannabe Channel swimmers (married to that one, actually, and at age 45 he doesn’t see why he shouldn’t acheive that with time and training.) None of the things above would have been likely with any childhood pushing around. These are things they’ve done for themselves as adults.
You are in your 20s. Do you have children yet? It’s not impossible to change direction if you have, but certainly easier. You have so much time and ability to drive your life in the direction you want it to go. Decide what matters most to you and then go for it. Don’t look back at the things that never happened for you, that will drag you backwards. Look forwards and take it from here, now.