It's definitely pretty easy in the US. I've met people who retired from real estate because they wanted to be high school teachers, accountants who become restauranteurs, etc etc. If you have the ability, it doesn't matter what your past is. That said, one reason it's so common is because more and more people want to run their own business as they get older. Sometimes people like the lawyer in the original post are just saving up so they can chase their dreams...of selling hot dogs.
I'm looking forward to changing careers a few times. Right now I'm in accounting, but I want to try marketing, teaching, and running a bar.