Attending university as an adult freshman After graduating high-school, I did not attend a university and instead decided to enter the work force. Eight years hence, I would like to pursue a university degree but I am not entirely sure which credentials are important for an adult freshman. Are my high-school transcripts still relevant almost a decade after the fact? Do universities consider professional experience in their admissions process (I have 6 years professional experience related to my intended field of study)? Do I have a shot at competitive university even though my academic history is essentially nothing? With sufficient review, I expect I can achieve high standardized testing scores, but I wonder if that will be enough to compensate for such a huge gap in my academic career. 

The crux of my question is: how should an adult freshmen prepare before applying to a competitive university?
I would say definitely - it's worth a try. I am studying at a university that usually ends up high in rankings (although it dropped quite a bit now because of student feedback), the oldest person in my course is about 9 years older than the rest of us. He spent around 5 years after school working (he had already worked there during school) and then two years ago took some additional courses at an adults' school and applied here. What should I say, they happily took him and he's constantly getting the best grades in the year.

As you mention that you have professional experience related to the subject, I think you may find it interesting that his years of work were completely unrelated to the subject he is studying now. I think that's a good factor as well.

I think in such a situation I would put much more stress on what I have done since school rather than what I achieved at school. With such a long gap, what happened at school should be practically irrelevant unless you can demonstrate what strengths you have taken from school that you still maintain today. This should go by the same reasons for which certificates such as IELTS or TOEFL are not accepted more than two years after getting them.

Finally I should add that this is a UK university. Procedures in the USA may of course be very different and so may your chances - but in any case it's worth a shot. It can't get much worse than having your application rejected can it?