Where do I synthesize dragons?

I've reached Castle Level 16 while doing hidden and time quests. I've been leveling up my dragons in between, but I heard you need to synthesize dragons over level 50 to transition to senior, yet I can't find how to do it anywhere. Is it not available yet because my castle level is too low.?? I currently have two dragons of the same color at level 50 and one of a different color.