It isn't easy to tell the age of a dragon by size and weight alone. All dragons grow at different rates, even ones that are all fed regularly and well. BDLvr's dragons grow fast but I wouldn't take them as being typical for dragons.
13" and 16" are sizes typically seen in juvenile dragons, no more than 6-8 months old with a few very slow growers being close to a year before passing the 16" mark.
However, if they weren't fed enough during their first year, dragons may not grow the full length they could have, with regular proper feeding and care. Genetics can also play a part in a dragon's adult size.
My suggestion, is get them on a regular feeding schedule with plenty of wholesome greens every day and some insects added as well. If they are able to grow more length wise, daily feedings will give them that opportunity. If they are over two years old, they may not grow much length wise, or very little but will definitely fill out more. Just make sure they don't get fat.
As for breeding them in the future...I don't recommend breeding them at all, especially if they don't get much bigger than they are now. Though smaller females can still produce eggs and have healthy young, if the smallness of size is due to genetics (ie maybe she isn't pure bearded dragon, (rankin's/inland dragon hybrid) 13-14" would be normal. However if her shortness is due to inbreeding or genetic defects...and if the male is related to her (they could very well be clutchmates....hard to tell without knowing the history of these two dragons and if they were bought from the same source at the same time or different sources completely, or at least different bloodlines).
I also don't recommend breeding even healthy unrelated dragons just for something to try out. Breeding bearded dragons is a responsibility not just a cool thing to try. Hatching out all the eggs and having babies to raise and find new homes takes a lot of time, money and space. Not to mention dedication. TO breed just one time, the cost to set up properly isn't worth it and you do have to find homes for these babies. Don't assume you can sell them all off to the nearest pet store as many won't take babies from anybody walking in with them.
If you just want your sons to experience what it is like to breed reptiles, get a couple leopard geckos or crested geckos. These are easy to care reptiles that are easy to breed and don't produce huge litters. It is much easier finding homes for about 8-12 babies than it is for 50 babies. Also, don't need as much space to house these babies till they find a new home. Nor do you need huge amounts of small crickets to feed them through those first couple months.
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PHLdyPayne