Well, if you want to ride on one properly, you've got to train it first. Getting a full grown one that's able to hold a full grown man right away would probably mean they're wild and not used to being rode. Ridden. Roden.
Anyway yeah, seriously. Think about it. I don't want any of you guys like, getting burned up by fireballs or like, totally having to defend yourselves from dragon attacks, although that would make for a most epic concert.