Training of dragons will start at town hall level 7 provided that you have upgraded your barracks to level 9.
A dragon is a mighty flying unit that can attack both air and ground targets.
It is also one of the most commonly used troop in air attacks during clan wars.
When you have just obtained a level 7 town hall, the next thing you would want to upgrade is your barracks so that you can start training dragons.
The level one dragon will cost you 25,000 elixirs to train.
Training a single dragon will take 30 minutes to finish.
So it is best if you upgrade as many barracks as you can to train dragons simultaneously in preparation for a clan war.
When you upgrade your dragon in the laboratory, the training cost will also rise.
The higher the level of your dragons, the more expensive it gets to train the dragon.