How do you know how long to age your beer?

When reading recipes they always tell you how long to age your beer.  When you are making your own recipes what guidelines do you use to figure out how long a beer should be aged before consumed.

Generally, I've found that 4-5 weeks from pitching is usually good for the ales I make.  Lagers go 2-3 weeks in the primary and then another 4-5 weeks lagering.  I have found that my beers improve in the keg for another month or two after kegging, depending on the beer - but I usually don't wait that long to start drinking them.  Some specialty types - like belgian wit beers can take a lot longer.  Ask about a specific style on this site and you will probably get an answer.  You can also google the style and probably come up with a dearth of articles that will help.

All my ales are done in 2 weeks.  I start drinking them starting with the hydrometer sample and then after they are kegged and carbonated.  Usually a couple days though I don;t wait for carbonation to be at full volume.  Sometimes I fond a like a beer with less than my norm 2.5 or so volumes.

I continue to drink it as it reaches it's peak starts down the slope to losing some of it's flavors and character.

I find no reason to wait.