I´ve been playing World of Xeen (MM4 and 5) for a while now but have yet to get very far, planing on doing the two worlds one at a time.
The thing is that my characters are allready at age 21, is there a way to lower there natural age or do I have to make new characters later in the game?
Is there a way to level these new charecters to a good level, now when most of the easy monsters are gone?

To get your natural age high enough in World of Xeen for it to actually become an issue you have to try... really try. So don't worry much about natural ages, as it's not going to become an issue unless you specifically try to make it an issue. Unnatural aging, on the other hand....

Like our friends above me have said, you would have to go far, FAR out of your way to age your characters enough to make any difference. You should, however, keep an eye on your characters' ages, because some creatures and events in the game will age you unnaturally. That can become a problem if left unchecked, because advanced age causes stat penalties, and thus can kill you just like disease or poison.
Happily, there are ways to reverse unnatural aging, and once you discover them, you can repeat them as often as you need.

There are temples and pyramids on the Cloudside that can only be opened with keys from the Darkside and vice versa. Artifacts and clues acquired within these "bonus" dungeons lead you to a third ending that can only be achieved by completing both games.
A word of "caution", if you beat either game first without setting foot in the other, you'll find it ridiculously easy when you do switch over. Going from Clouds to Dark isn't really so bad, you should only be level 20 and only the first few dungeons will be too easy. If you go from Dark to Clouds, you'll be up around level 100 (at least) and will basically be walking gods around Clouds. The best is to use the pyramids to flip back and forth between the two, at least for the first few dungeons of Darkside. Then finish Clouds and come back and finish Darkside. Then complete the bonus dungeons and complete the World.

I played MM4 first back in the day, and then went out and bought MM5 and worked through that. While the first town was extremely easy, I seem to remember the difficulty catching up to me pretty quickly, so I wouldn't worry about doing Clouds first and then Darkside. But like WizardStan said, you'll get to a MUCH higher level in Darkside so doing that first really will make Clouds too easy.
I'm taking a break after finishing MM1, but after I've played through MM2 and MM3 I'll finally return to Xeen and see what it's like playing the two games together. My guess is that being able to warp to Darkside will help in the early game as it gives you a second easy town and maybe some easy dungeons (my memory is a little fuzzy on that) to do if you're having trouble in Clouds. I doubt I'll do too much of Darkside before deciding to just tackle the rest of Clouds though; I feel like Darkside really ramps the difficulty up so you can take your party to much higher levels. I remember the Darkside manual going over some strategies related to the steeper difficulty curve of that game. You'll likely find most of Clouds to be easier, but by all means switch back and forth and try things out.
Also, the third endgame that has you going between Clouds and Darkside is pretty awesome. You're in for a treat.

I just finished darkside of xeen so it´s time to link the two worlds together, this is really a great game!
The thing is I´m "only" at lv 65 and age 25, with almost no gold, so I´m going to start to make some money by enchanting items and sell them. At what age does the stats start to go down?

I think you start getting penalties at around age 35, but they're very minor at that point.