Well for the first few parts of Konquest he was in his younger form. If its a part thats on up in the game, then they are probably using a cheat device, such as an Action Replay.

I like the part when he is his fourth self. I just don't get how he changes his clothes during the aging. Like in the pool, how he goes from a blue shirt to a yellow one. WTH?

Someone who was unaffected by the pool probably came by and decided to mess with him taking his clothes. Then when everyone got tired of seeing his naked ass standing there, they put some new ones on him.

OR

It could be just one of those things. Like in Legend of Zelda: Ocarina of Time. Whenever Link pulls the Master Sword from its pedestal, he is sealed within the Temple of Time until he is old enough to be able to weild the blade and take on the next part of his quest. In the process, he gains a larger tunic, undershirt, stockings, and some big brown gloves. Oh yeah, and an ear ring too.

It's just showing how he ages. It may sound and look confusing, but unfortunately, the change in clothes is just showing the aging. Remember, a lot of things will never make sense if you try to explain, if you just go with the flow and realize what is actually going on, then there's nothing to be all "WTH" about.

Same with Armageddon's introduction movie. On Youtube there's a video that shows a lot of things happening that we will never pay attention to, except in this video. Such as, there are certain characters who does something to either one person or a group of people and within the next frame they're there after oh lets say, getting knocked off the pyramid by Kahn's hammer.

It's just like trying to decipher how this is possible, but if you just go with the flow, it will most likely make sense to you.

Then how come shujinko grows old and everybody else stays young?

Some of the characters are immortal, meaning they won't age because they don't die. Take Raiden for example. He may have changed his clothes over the years but he still stays the same "age". Scorpion as well, because he is a spectre, so he is already dead.

Also if you notice, many of the more important characters follow this pattern of aging, and you don't really meet anyone when you are young, then really old. Unless you consider some of those random battles. Those aren't really a canon part of Konquest. Not everything that happens it is true other than Shujinko's interaction with main characters like Raiden and Bo Rai Cho, who I would assume dosen't seem to age either because they had not took the time to make separate designs for him, or that he ages differently because he is from Outworld.

Becuase in the gaming world, only one can age.

J/k, but what Smoke01 explained is correct.

I wish I was in the konquest, then I would neveer die.