I am curious to know if a vehicle's idle rpm will increase/decrease over the period of time of its usage.If yes why?
Yes. Because the atmospheric conditions change constantly and load on the engine changes constantly, the proper volume of air to let through the closed throttle body to maintain the correct idle rpm changes all the time. If this adjustment isn't performed, the idle will raise or lower depending on the ambient temperature and atmospheric pressure, how many accessories you have running, battery charge level, etc. Manufacturers have developed various schemes to solve this problem, most of which can wear out over time.
At the simplest end of the spectrum is the lowly throttle body screw- a screw blocks the throttle from closing, leaving it slightly cracked open when "closed". By turning the screw, we can adjust how much air gets through the closed throttle body. Although this approach is crude and normally requires periodic adjustment (as seasons change, or the car moves between different altitude locations) this is the least prone to wearing out. However, the throttle body can become dirty over time, decreasing the volume of air that gets through the small crack. To fix this, clean the throttle body or adjust the screw to raise the idle.
Some cars adjust idle with an electrically controlled valve next to the throttle that can wear over time or get clogged with dirt.
Newer cars with drive by wire throttles will adjust idle by adjusting the throttle itself. These are least likely to be affected by dirt, since they can compensate for dirt by opening wider. And any malfunction will also be easily detected because it will affect the behavior of the car off idle as well.
Absolutely. Since you don't specify which type of engine or give any specifics, electronic fuel injected engines usually have either a Idle Air Control (AIC) valve or if using a wire driven (versus a cable driven) throttle body, can vary the idle as needed to ensure the engine continues running during warm up (after startup will have a high idle), then will throttle it down after the engine is warm to the normal state. With an IAC, whenever the throttle plates are closed, the IAC will open how much ever it needs to ensuring proper idle stability.
No, idle speed doesn't change as a function of the vehicle's age. As Paulster points out in "normal" operation it will fluctuate based on environmental conditions (temperature, fuel quality etc) as the ECU seeks to maintain a correct AFR (or in the case of old-school engines the driver does the same thing via a choke etc)
Things like wear and tear on the air/fuel filters, injectors, fuel pump etc can cause effects on the AFR and result in the ECU adjusting idle to compensate in the same way but not usually to the level where you would notice, if it's getting to that extent then something's wrong in your servicing schedule somewhere!