Adblockers on iOS require the 64 bit A7 SoC which was released with the 5S. I'm not sure if there's a valid reason why they couldn't make it work on 32 bit too, whether it was just extra effort or whether they'd have had to make technical concessions, but the way they implemented ad blocking within Safari is different to other solutions (e.g. the side-loaded ad blockers that 05 mentioned) so it's hard to be sure.
The first 10 minutes searching didn't yield any results. Needs jailbroken device and then you can fiddle with your device's host file is what a lot of comments say.
My iPhone 5 won't let me install Ad blockers. Which is kind of funny. I can't think of a good reason why this feature depends on the hardware.
Also, it makes Apple's statement that they care about their users' privacy quite hypocritical.