Why does the power light on surge protectors flicker
What you took from the discussion is roughly correct. Neon lamps wear out. The brighter you make them (the lower the value of series resistor) the shorter their lifetime. 

For example, the ubiquitous NE2H is rated for a "useful life" of 25,000 hours. If it is powered 24/7 that's less than 3 years. For lamps designed for non-indicator purposes (where electrical characteristics are important) 2,000 to 7,500 hour life was commonly quoted (only a few months 24/7). 

As they age, the firing voltage increases. It can be decreased by ambient light and by radiation. There is a very small temperature effect on firing voltage (about -40~50mV/°C). Some lamps have a radioactive additive to make the firing voltage more consistent. 

The firing voltage rises during the life of the lamp. When it gets high enough it will no longer reliably fire (so the aging rate decreases). It can spend quite a bit of time just firing now and then when hit with a bit of radiation. Once it fires, ionized gas is left over between cycles so it will tend to stay on for a while.