I started when I began going to WDW as an adult, but I really have very few compared to most collectors (I think I have around 50, haven't counted in a long time).
It is very easy to get addicted, which is why I have rules, LOL :
1) I specialize. The only character pins I purchase are of the Witch from Snow White (my favorite) or Maleficent. If you collect the generic character pins like this, I highly recommend specializing. If you don't, and you just buy every cute pin you see, you will go broke quickly as there are thousands of them.
2) I collect attraction pins from my favorite attractions - Haunted Mansion, Star Tours, etc. I don't need all of them, but I allow myself to pick up a few of those that really stand out to me.
3) I collect "event" pins. For example, the only one I bought at Disneyland last year was the 15th Anniversary Pin for the Indy Ride, as I was there that day. Other pins like this - opening day pins (I'll be sure to get a Star Tours one), or the ones for MVMCP and MNSSHP.
Even with just those categories, I still manage to spend a bit. Best bet is to set some ground rules for yourself, before you become one of those people covered in pins with a maxxed out credit card.