In addition, it is fairly difficult to pop older animals, since they have a lot more muscle in their vent area than when they are young. I do not see that popping would harm them, unless you do it wrong. I have popped older ball pythons with success, but I have had to probe others.
Probing is not difficult, but is much more likely to cause damage than popping. You have to make sure that you use the correct probe. One to small may pierce through if you push too hard. With the correct probe, insert into the vent and rotate back and forth through your fingers while gently pushing down. Once you feel it stop, place your finger on the probe to determine the depth. Pull out and put the probe against the scales to determine the scale count. Males will have more scale count than females. Females are somewhere around four, if I remember correctly, and males are around eight. If you do not feel comfortable doing this, ask someone with experience to help you.