I was 21 (in 2009) when I started Go and made it to EGF 6-kyu in a year without putting in any structured effort, except for the last month (
http://www.lifein19x19.com/forum/viewto ... f=12&t=670). But I regulary played games and "reviewed" them with friends (who were my usual opponents). This was the time, when I was weaker than almost everyone in my Go club, so every game was a teaching game.
Shortly after that I started my study journal (
http://lifein19x19.com/forum/viewtopic.php?f=48&t=977) but it turned out to be very bad and I stopped playing Go for a few months every now and then, so I didn't really improve over the next year and just got 2 ranks stronger.
In 2012 I still was EGF 4-kyu. I also credit this to having no real plan for improvement, I just solved a bunch of problems but with no measurement, when I finished a book I put it away. No matter how good I was at solving it. And I stopped playing Go for a few months again.
Then I started my cylce of problem books but some weeks later I got drawn towards Starcraft 2 and stopped doing anything for Go. A month later I continued the cycle and also replayed Games of Shuei with quite some effort. Another break came (still Starcraft 2, Guild Wards 2...).
In the beginning of 2013 I came back to Go, this time more structured, with a bit more endurance (around four months?) and I also registered for an online Go league for two months. Then I stopped again (albeit playing as a EGF 2-kyu in this year's tournament). And now I'm back in full pursuit since two months, bouncing on and off Shodan on KGS.
Self study for me is a path with a lot of backtracking. I most likely wasted a year with my initial study plan (first post in my study journal) because it was just plain bad. Then I wasted time reading theory books, when I obviously couldn't spot common shapes. I also wasted time "studying" Josekis, it was not much time but still. Of course stopping to play Go every now and then does not really lead to steady improvement, too.
Right now, I think I have found the right mix for me. Mainly solving easy to intermediate problems (but check myself through the score in the cycle), replaying one professional game a day (when I have the time), play around five games a week and ask a stronger player for reviews of the lost ones.
I think having a teacher is easier and can lead to a more focussed study plan and when money wouldn't be an issue, I think I would definitely get a pro teacher.