Dragons as stated with the ability to fly when young and with the ability to produce flammable gas are leading a quite high-energy lifestyle, which would require a large energy input. As they are carnivores, this dictates a certain lifestyle.
Cold-blooded carnivores are ambush predators. They excel at sitting still for long periods waiting for something unwary to happen by that they can catch and eat. Since they don't generate their own body heat, their baseline metabolic requirements are quite low.
Warm blooded carnivores have a higher energy requirement simply due to the fact that they must maintain their body temperature, typically above the ambient air temperature. This means that they must be more proactive in finding food, to the point where they actively hunt for their prey.
For a creature that must expend large amounts of energy flying and generating flammable gas, it seems unlikely that a cold-blooded metabolism could provide the capability to obtain sufficient energy input for their lifestyle.
This means that dragons must be - to some degree - able to regulate their own body temperature independent of the environment so as to be able to hunt in weather conditions that would not favor a cold-blooded animal.
All animals, whether homoeothermic or poikilothermic, require more energy input per unit mass when small than when they are large, and the gradients are identical, i.e. by increasing mass a certain amount, the energy requirement per unit mass goes down proportionally, regardless of whether the creature is homoeothermic or poikilothermic.
In addition, marsupials have lower energy requirements per unit mass than placental mammals of the same mass, but not as little as a poikilothermic creature, so it is not beyond the bounds of possibility for dragons (or dinosaurs) to be homeotherms and still have a lower energy requirement than a placental mammal.
Other strategies employed by mammals to reduce energy expenditure are sleep (practiced by cats) and hibernation (practiced by bears and rodents). The ability to spend long periods sleeping or hibernation would fit well with some popular representations of dragons.
In conclusion, I would suggest that dragons are most likely to be lower-metabolism homeotherms (i.e. warm blooded) than placental mammals with a propensity to long periods of sleep and hibernation.