6 years of experience or 6 years seniority in the company? Because SWA requires 2500h total and 1000h turbine PIC preferred (more or less mandatory, unless they are desperate for pilots). So it will take quite a while to get 1000h PIC, before one even meets the minimum requirement to join. Oh, and add the time and the cost of a 4-year degree to that.
Having a glance
here, you can see that only 219 of their pilots are aged less than 40 (will be 65 in 2046 or later), so it's not like year 6 pilot is 25 - most likely it's double that age.
I bet if you are in the left seat of a European LCC by 25 on a 6-figure salary and
invest the money right, you won't be far off from the average USA pilot - remember, not everyone makes it to the majors there at the age of 23, and a lot of people are stuck in regionals for life.
And at the end, it's all sticks and stones - they've got the right to live and work in the USA, most of us don't, so those jobs aren't accessible anyway.