Not everybody is a mobile phone enthusiast, and majority of the people out there can only afford to buy a new phone only once every couple of years. So, by the end of the first 12 or 18 months, the phone that was so blazing fast when you first got it, might just start crawling.
There can be thousands of reasons why an Android device slows down over a period of life, including poorly coded apps, less free RAM etc. However, majority of the times, the slowness can be fixed by following a few simple tips and tricks.