It really depends on their size rather than age. The earliest would be 8 months of age but much later with small individuals.
You just have to feel comfortable with handling them before and when the probe is inserted, i.e. not to bend their backs at cloaca and not to hold the tail too firmly. I always spend some time just handling them, running my fingers around and over the posterior until they relax. The right size probe and lubricant are essential.
Another point - I always rest the probe in a glass of warm water between probing. LOL No one would like a piece of cold steel up their ....... :shock: