A lot of the factors you have stated for age/size/health are VERY IMPORTANT in when any animals should be breed.
I think that some of the people on here are concerned when they look at the "home hobbiest" that has a pet dragon 7 months old, always seeking help help for various parasites, worms and other illness and in the next breathe are talking about breeding them in a few months, cause a popular breeder says they have breed at that age or their site shows dragons younger than that breeding.
To equate a breeder that has been doing this for 10-20 years with a new owner of their first sub adult and breeding is like comparing me to Arnold Schwarzenegger in an action film.... I would be in over my head.
Professional breeders, this is their income and life, with LOADS of experience and knowledge to care for the problems that may come up and they know the risk in inbreeding (if they do) and are prepared to deal with the results should they not be good. Still some have been seriously hurt financial by practicing methods that were long accepted and common in the reptile industry and they DO adknowledge they have made mistakes and are seeking answers.
A new owner buying siblings or even unrelated reptiles and thinking they are going to breed at a year old or younger, produce and care for the parents and for offsping the same is dreaming!
I've been in large breeding facilities, excellent lighting or UVB access, supplements on scheduled basis, balance diets, IMMACULATE conditions and the best of medical care, whether by the breeder who knows what they are doing or a quaified vet. And they make sure those breeders are in prime condition. Yes, with this and known genetics(which 99% of new owners do not have a clue of) you do get dragons that are mature at younger months..... but rare is a owner with a pet for the first time going to have all those stacked in his favor.
Dragon breeding should be left to professionals or those committment to a purpose in breeding who have the experience to know what they are doing, the dragons they are breeding (history) and the dragons should be in peak health... unfortunately that is not the case in many private breedings, there are exception, on this forum there are some that meet that, but they are rare.
Final conclusion.... its the owners choice, they should just be prepared for the potential end result and unless they are well educated, it could be a poor one.