Phone overheats after powering on

I've seen this with plenty of smartphones,usually after 1-2 years of use.The phone overheats for a minute or 2 before cooling down,but it's performance doesn't differ from a non-overheating phone that has been powered on. Is this simply due to the natural wear and tear which causes a CPU overload for the first few minutes,or it it because of the failing/malfunctioning of a particular component in the phone/(which would in turn, adversely affect its battery life)