If it were me...
I would have a symbiotic relationship too. Perhaps the bond that develops between dragon and human is what gives dragons their intelligence? Wild dragons are almost always mere beasts, cunning, certainly, but just powerful animals. The ritual that binds them allows for the boost in intellect. Then you have to decide what happens when the human dies...
I would almost certainly make a prestige class for the dragon riders. Probably 3 or 5 level one that focuses on their empathic bond, boost to INT for the dragon, and perfected flying / fighting coordination. It would be designed so that it would give an edge to a dragon rider, not a requirement to be one. Say, 3/4 BAB, Good Reflex, Bonus to Ride equal to twice class level, d8 HD... other stuff.
Adventures would involve perhaps a 1st level adventure that lets them "earn" their place as a dragon rider or otherwise shows what qualities are needed to be a good one. Then there is the bonding. The second adventure would be while they are still too small to be a mount; a training mission perhaps to strengthen their bond. Next mission would be their first actual flying adventure. There would be some significant downtime between these adventures, actually. Enough perhaps where I would consider having the first adventure be at 1st level, then 3rd, and then true riders at 5th. Normal progression would follow from there.