What is the voltage from which a normal humans starts feeling electric shock?
Background

We don't really feel a voltage, we feel a current (not "amperage"). Because a battery can be approximated by a voltage source, the current that pass through the body can be calculated by Ohm's law, I=U/R, where U is the voltage from the battery and R is the skin resistance.

The problem is that you generally have no idea about the skin resistance. Dry skin can have up to 100k or more, but there is no lower limit. If you stick sharp probes straight through your skin, you'll get an almost perfect conductivity.

What is the lower voltage?

Remember that we actually feel a current, and the current is related to the voltage and resistivity. It doesn't take a high current for the human body to register it, and instead of reading through Wikipedia for you, I simply decided to test it.

The part of my body with the least resistance is my tongue. I hooked a voltage source up to my tongue and I could easily detect 1 volt. Less than that, maybe, but I would have to have someone help me with a blind test.

When trying the same probes on my slightly moist fingers, I could detect a voltage of around 15-20 volt.

What are you doing wrong?

I suspect that you confuse earth, ground, and negative. A circuit must be closed to flow. A battery is not a bucket of water that you can pour electrons from, even though a lot of teachers and texts out there apparently wants you to believe that. There must be a path of low resistance between the positive terminal of a battery and the negative terminal on the same battery for any current to flow from that battery.

If you connect the positive terminal (+) on a 12 volt battery to one side of your friend's tongue and then negative terminal (-) on the same battery to the other side of your friend's tongue, I can guarantee you that he will feel it.

I can, however, not guarantee that he will still be your friend.