I had that exact thining pattern when I was 19.
It worsened with time, but not too much. Now I´m 25 and I have better hair than those days, because I´ve been using the big 3 for 13 months (minoxidil for nearly 3 years). Thinning was in the back majorly, but when I was 23/25 I started noticing how it was extending to the front via the middle part. Luckily I started my actual regimen and now it thickened my hair to the point that I´ve got no noticeable thinning apart from the swirl, and even there i have enough hair to decently cover my scalp, wich I haven´t been able to do since I was 22. Still I have not matching density in the top than sides/back.
Lately I´ve been thinking in our thinning pattern, female like. It is said that is the pattern associated with hypo-thyroid/adrenal fatigue diseases, and I have many symptoms associated: thinning eyebrows, no dream recall, inability to sleep til late hours, low body temp in the morning, anxiety and inability to deal with stress, panic attacks, weakness, fatigue, bad nails and hair, never having breakfast, cold hands and feet, mood swings, etc...
I´m telling you this to see if you have any of those symptoms, just for curiosity, maybe you can extract any useful info, too.
I´m planning to start a zinc/b6 supplement approach to the matter, as it´s said that deficiences of those could cause adrenal fatigue and many of the symptoms I´m experiencing. I´m not holding my breath for any hair regrowth, but I have the feel that there´s something to do between adrenal fatigue/hypothyroid and hair loss in female-diffuse vertex pattern.
But I´m gonna keep using the big 3, as I´m getting benefits from it. My plan is to make a general treatment, getting the adrenal-fatigue cured and male pattern baldness under control, because I think both mixed are the cause of my thinning pattern, that, as I said, is exactly the same as mine.
Your thinning could be just male pattern baldness (if you don´t experience any of the things I listed above, hypothyroid related), and you can treat it succesfully using the big 3 (more efficient in the vertex area), but stay alert to any alternative cause.