Personally I feel dragons do quite fine on their own. As you have a male, keeping him alone is perfectly fine. He certainly doesn't need company. Once fully mature he will have some urge to mate but with no females around he will be fine.
If you do want to house them together, BDLvr explained most of the housing concerns.
I really don't think it matters too much the ages of the dragons, the size does have some impact though. IF there is a large difference in size, dominance issues can arise. However I would go for a female that is older than your male. At least then if they do breed she is more mature and grown to handle the egg bearing. Of course by the time quarantine is over, he will be a few months older as well, so basically you want your female to be at least 14 months when you put them together. It is also a good ideal to introduce them on neutral ground (ie living room floor of your house, under close supervision by you).
As you do not want to breed, any eggs produce, just put them in your freezer for several hours or over night, then toss them. Flushing down the toilet will work to. The ideal is you dont' want to just toss them in the garbage as you do not want them hatching out in some landfill to either suffer a slow death of starvation or freezing etc. Or to actually establish a colony (the government really does not like non native species getting a foothold, even though bearded dragons are not as destructive as other reptiles careless owners have let loose, such as burmese pythons, green iguanas and others in Florida). Besides, properly disposing of the fertile eggs is just being a responsible and caring pet owner.
Other issues in keeping dragons in the same cage.
Competition for hides/food/basking areas. BDLvr explained ways to get around this. Multiple hides, hot spots, more than one food dish etc.
Spread of Disease/Parasites: If one gets sick, the other is almost guarenteed to be sick too. Same with parasites. Not to mention, unless you actually catch one in the act, you wont' know who's poo is who's, so if parasites are found in the sample, both dragons will need to be treated. Then again, even if you know which dragon who's poop fecal was positive, it is likely the other one will have parasites too, or could still catch it from the infected one and even reinfect him, so both will need to be treated to ensure they don't cross contaminate each other.
If anything is thrown up, you won't know which dragon regurgitated or pooped, so hard to keep track of who did what on a daily basis.
Stress and fighting. IF the dragons don't get along well they can fight and severely injure each other. Even long time cage mates can suddenly have a bad day and you discover one has a missing foot or worse. If they only slightly hate eachother, they can both be stressed by the others presence, which can lead ot secondary medical concerns, such as parasite, coccidia flare ups, lack of appetite etc.
Many dragons do live together and never have problems at all, but some do, so there is that risk. Also you must be prepared to separate your dragons at any given time when you do notice a problem. If the male is obsessively mating your female, she can be over stressed and stop eating etc, so you may have to take him out or her out of the cage till breeding season is over. If they suddenly fight or one is injured you will have to separate them etc. So you pretty much need to keep an appropriate sized cage handy just in case so its almost better just to keep them separated all the time.
If you do decide you want to try keeping a male and female together make sure you a large cage, at least 5'x2'x2' for a pair. IF you want three, then I will give an extra foot of length or width. 2' of height is all you need.
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PHLdyPayne