I co-founded my first startup when I was 16, as soon as I reached the lower age limit for being a company shareholder in the UK - I'm not from the UK, but I wanted it and so I found a way. If a 16 year old kid with acne and 300 euro in their wallet can do it, any adult professional can.
Yeah it's a load of risk, you work crazy hours, the stress is insane and sometimes you end up paying instead of earning - so don't be surprised that nobody will share the upside if you don't share the risk. It's not for everyone. But anyone can, if they actually wanted.