The air charge in the tank can only be measured after you drain all the water from it.
Life of the bladder will vary depending on what is in the water. I've heard others here say that 8 years is the average life of a bladder tank.
If you have an air pump, the bladder pressure should be set to 2 psi less than your cut-in pressure. Make sure you use an accurate air pressure gage, not many of them are.