when to get new stylus

Six years is a long time for a stylus unless you only play a few records a month. Generally a diamond stylus has a life of around 1,000 hours. Six years of average listening would probaly be a lot more than that. One of the big advantages of the Shelter 201 is cost of ownership. A replacement stylus from an authorized dealer is only 100 bucks, which is way cheaper relative to the cost of the cartridge than most other replacement stylii in that price range and it's way cheaper than a retip for a moving coil cart. The more sophisticated MM replacement stylii are a lot higher too. Even the replacement stylus for an Ortofon 2M Blue is twice as much as the stylus for a 201. Get a replacement stylus for your Shelter 201, snap it in, and party on!

Tough question.  How long do you estimate you have used it in hours?  Have you washed your records on a quality cleaning machine with quality fluid and placed them in new quality rice paper sleeves?  Do you periodically clean the diamond tip with a zero dust system or disc doctor (brush) system?  Do you use a quality Hunt carbon fiber brush to remove light surface dust prior to playing the record?  Do you play the 'Cardas Frequency Sweep' record to keep your cart in top shape at least twice a year?  Have you verified your set up has not changed over time?  I've noticed a minor change in anti-skate can muck up the sound.  Have you verified the RPM of the record has not slowed below 33 1/3?     

An internet search states a diamond can last up to 5000 hours.  Note there are many different types of diamond shapes used by manufacturers.  Some may last longer than others.  I would try a cleaning prior to replacement along with the Cardas Frequency Sweep record.  Its a great tool to own if your into analog.  If that does not help johnnyb53 has answered your question.  Just age, not usage, tends to be hard on the suspension system of the stylus.  Ozone in the air can harden the rubber and change the carts dynamics....I'm not a designer but have been purchasing high end audio equipment since the 70's.  A little tribal knowledge has seeped into my brain over time.