Mine went down LOADS when I turned 21, so much so that I bought a MK5 lol. Saying that alot of companies (and I mean a lot !) still gave me silly prices. I eneded up with Elephant (dont get me wrong, its still fooking expensive lol). The cost of insureing my MK4 being 20 and being 21 also went down loads aswell.
I was also looking at Bimmers before deciding on the MK5 and I could get cover on 328i withtin my price range, so from my experience yes, it goes down nicely at 21 (for me anyway).