Without know more about the product, how it was maintained, type construction, condition of construction, and installation it would be impossible to give you a correct and proper answer. Pictures would probably help tell us more of the story. But in a simple answer yes a properly maintained, installed product in its recommended application should last more than 20 years

The leaking could be due to a variety of things and is impossible to diagnose without more info. A picture would definitely help.
On Marvin, it is definitely one of the top offerings available in wood windows/doors. That said, any wood product is going to require maintenance if you want it to last.

...or in other words, ditto to mwm's comments ;)

I think is about sums up your post....

wow, I just spit coffee thru my nose and now to clean up my computer!

lol, I actually wrote that a couple hrs earlier and never hit submit, but that's ok... leave it to WoW to draw from his vast library of cuddly kitten pics ;D

I'll try to take some pictures and post. It's clad on the outside wood on the inside. What type of maintenance is required for this type of door? What is an incorrect application for a six foot French door with one side that opens? It seems pretty basic.

Pics would definitely help, but the primary maintenance would be to maintain the finish and seal on the wood.

Okay - I think that could be part of the problem then. I think there could have been some issues with how well the hinge sides of the doors were finished initially. At this point I am planning to replace with Marvin sliding French doors and hopefully the person doing this work will get it right. What do I need to look out for?

What are your thoughts on sliding vs swinging?

HomeSealed...are you a felinist?

That is a hate crime in today's day and age. Be aware that I may report you.

With doors you want to make sure that the threshold of the door is set in sealant and sits solidly on the sub-floor. It's common for this to not be the case, and eventually the threshold bows down and creates a gap where dirt collects and holds moisture. Don't know if this is what happened to you, but it's something to watch for.

And like the other guys said, make sure that the bottom of the door itself is sealed properly from the get go. Also, personally, it doesn't hurt to remove the hinges and seal behind them as well.

New windows more than 20 years?

You must be kidding.

Gibby - To answer your question of swinging vs. sliding, there is no difference with Marvin as far as design pressure (performance) is concerned. There are more moving parts on the swinging doors thus, more to go wrong down the road.
As for the leak on your existing door, where is the water coming in? That can usually indicate if it is product or installation issue. Highly unusual for a door to remain dry for nineteen years and then suddenly leak without something happening (broken door sweep, etc..