At what age can a dragon carry a rider?
Whatever age they reach the size category above what the rider is, or the same size category if the rider has Undersized Mount.
the age that its a size category larger than the rider, and it has the weight allowance to still be in its light load once the weight of the rider is accounted for.
you have to do....math....
As soon as it has sufficient Strength to carry the rider. It won't be suited for the rider, though, unless it's one size category higher (feats or other abilities notwithstanding). Riding an unsuitable mount penalizes Ride checks and prevents some uses of said checks (e.g., fast mount/dismount).
thanks all...that's perfect
That said, I would expect most dragons don't wont to submit themselves to being ridden. Even the good aligned dragons are very prideful about being dragons.