Riddle Me This GD

Why does everyone say to not play ranked once you hit 30? So you'll be in bronze for 6 months...either way you are going to wait until you improve before getting into silver.

I have never played ranked. I've been in League for over a year and my MMR is high gold now. So I am about to start ranked and I suspect I will land in gold or silver at the very least. And if I started ranked right away a year ago I wouldn't be in gold today?