I was 13 or 14 when I joined back in '07.
I guess what has changed the most is the cash shop. I remember when it was just MC's for cash; but I really don't remember when it had became cash orientated. Of course, the layout change is huge too. Shortly after I had joined, and was familiar with the site, it had changed. I was like, "Now I have to re-memorize everything!!!!"
I originally joined in '04, when I was 14. Hot damn, has things changed. No more rollbacks or sushi errors. But we did finally get zOMG, and now even the fabled backwings. Hell, I remember when it was a big deal that we finally got a gun item.
I feel like an old fart because I still, to this day, miss being able to get 10g for every single poll vote. There used to be entire guilds dedicated to pollwhoring. It was a beautiful time for a newbie short on gold...