I've been the same shoe size since I was 11 and I'm into my 20s now. I've been wearing the same pair of boots for almost 9 years. But I don't know if that's the norm though?
You can also look into finding a pair of used field boots. Some tack stores will sell them. I found a pair of used Ariats for only $40 on Craigslist. They were already broken in so they didn't kill my feet and all I had to do was add a bit of polish to make them look brand new.