Parents used to start discouraging this by a year- at least giving acceptable alternatives (teether idea posted above). But it's now every child I enroll has to be stopped of mouthing toys, regardless of age or habit. I have a 4yo and almost 2yo (who has his 2y molars) as the 'worst' offenders. I remove toys to sanitize, but that doesn't stop it.
I made special centers only the children who don't put toys in their mouths can do (smaller pieces) and that hasn't convinced the 4yo to stop. She just pouts about it.
The 2yo's mom admitted that she has the binky all the time at home, so I assume she is looking for the same oral stimulation. The 4yo though? C'mon!