Usually by 2 they stop putting things in their mouths. Well my 4yo still does now, but she's weird.
I remember when my oldest was sick one time and we had been in the ER and then went to a pharmacy to get her prescription. What was the first thing she did? Yup, she licked the counter. Eeeewwww!!! That's just kids. They do grow out of the sickness stage. To tell you the truth, my mother was super duper strict with me when I was little. I wasn't alowed to touch anything and she would disinfect everything. I was sick ALL THE TIME and even now get sick really easily.
As much as my older kids got sick all the time when they were little, by the time they hit 4 they had built up their immune system. Now they get sick for maybe a day or two and that's it. They usually pass it on to me and feel better. Hahaha!