I was watching Tron: Legacy, and thought for a moment that if Flynn is trapped within this 'Digital Frontier' that he created, wouldn't he be able to control his own appearance and aging? If not, then how exactly does time work in 'the Grid'? The only clue I can see is that time is measured in cycles, however I'm assuming that thousands of 'cycles' occur every second, making the amount of time that has passed within the 'Grid' to be millions of cycles that have happened between the time that Kevin Flynn is trapped and Sam Flynn finds himself in his dad's cyberspace. Last I checked, no one has lived for millions of years.
I quote from the Tron Wikia:
A Cycle (also known as a TRON Cycle or TC) is a measurement of time used on the Grid. Interpreting the dialogue of TRON: Legacy, a cycle is a fair amount of time, roughly equivalent to a year.
Kevin Flynn mentions that a millicycle is about 8 hours. The metric prefix "milli" denotes one thousandth, which would mean that a cycle is about 8,000 hours (for comparison an Earthly year, by the Gregorian calendar, is about 8,760 hours (twenty-four times three-hundred-and-sixty-five)). But that measure is speculative and non-canonical. For more info on possible time measurment, see Kevin Flynn's page.
The specially released TRON: Legacy tie-in magazine guide includes a timeline of the Grid, explaining that time moves faster in the system because its only limit is the speed at which electrons can move in circuitry. The guide states that one year in the real world equals about 50 Cycles in the Grid, which would mean that Kevin Flynn was trapped inside the Grid for roughly 1,000 years from his viewpoint. This would seem to be confirmed within the movie itself by Castor's line that Clu had been trying to obtain Kevin Flynn's Identity Disc for about 1,000 cycles.
Humans in the Grid seem to age at the same rate they do outside the Grid. However, the amount of time they experience in the Grid is much more than they would experience outside the Grid. Flynn was in the Grid for ~20 years, and he aged ~20 years during that time. However, he experienced decades, if not centuries of time passage during his time in the Grid.
It may be that after running for an extended period without external input the programs within the grid (including Kevin Flynn) started looping through the same states. There's evidence that nothing was changing until Sam entered the grid. Clu had completed his purge and had taken over. Kevin Flynn had moved out of Clu's reach and was using passive resistance to keep Clu from escaping the grid. Clu's attitude at the games was profound boredom, as if he'd seen it all a thousand times. So it's possible that everything in the grid had started looping through the same states, experiencing the same stretch of time over and over again without realizing it. Sam's entry broke the loop.
Just adding this as a point: doing some liberal, back-of-the-napkin calculations based on the original movie would lead you to conclude that the 20 years was subjectively 2 million years. This is not the case in T:L. However, that can be hand waved away with the fact that they were on a different system, and arguably Flynn could have set that himself.
You're thinking of an old man as you would think that he would die. On Earth, our bodies, unlike in the grid, are forced upon by gravity constantly while we age. That alone would improve his chances but clu is a robot life for AI, just like a toy aka a tool if you want to sound not like a child. So when you buy this robot 20 years ago, and it sits in a package not subjected to getting banged around, it looks new in the box 20 years later. Clu was never taken out of his, and I assume Flynn's body, though in a box, still degenerated because he was a human entity, not fully made up of AI parts. Forget the clyces, that roll plays little part.
If you want to blow your head up with some larger number you can take things to the extreme.
let us assume that instead of cycles referring to some analogous system of tron let them be Clock cycles of the processor (ridiculous but roll with it) so if we assume he was useing the most powerful processor available to run tron (if not many of them) then he would be running off a 120mhz processor which has 120 million clock cycles per second so on that assumption he would have spent 6575342 years on the grid per second