How long does horse manure have to "age" to be considered compost--and safe to directly plant in?

You can kinda tell. When it begins to be nice and crumbly, rich and earthy-smelling, and not manury-smelling.

Why would you want to plant directly in manure?

I dont..i want to plant in compost...and I'm asking at what point is it aged enough to be considered compost...I'd like to mix it with the other compost that I have started...and then plant in it. I have some manure that is very crumbly and can hardly tell it was ever manure...a few chunks here and there...but I have read where if it isn't aged it can damage the plants...and I want to avoid that.

You can test it. Mix some with garden soil half and half in a pot then plant a couple of beans. If the beans come up and grow good and have a good green color, it is good to go. If the beans don't come up or come up and are a sickly yellow color, or soon die, then you need to give it some more time or use it very sparingly. Plant a second pot with just garden soil for a comparison.

Horse manure like you describe can be spread thinly on the garden in the fall, and it will enrich your soil greatly. Just be careful to not get it too thick, as "horse" is pretty rich. One half inch to one inch is plenty.

I am thinking it is fine to add it to your compost. It shouldn't be a problem.

A lot of horse manure has sawdust bedding mixed with it. The sawdust takes some time to decompose. As it decomposes it uses up a lot of nitrogen. Of course the "horse" is loaded with nitrogen, so it works out well. You didn't say if yours had sawdust or not.

Aged manure is aged manure. Compost is a mixture of many materials. Simply because manure has been sitting around for a long time does not make it compost. That manure will need to be mixed with something else to be used for planting if only because of any E-Coli concerns you should have. Manure should never be used as a planting medium without first being composted with other material just because of that one concern.

Quite true. In my response, I was referring to manure that had been added to other compost ingredients.