From what I can see, Canoeing has an older demographic, more than half are male, mostly nice folks. Sea Kayaking is similar, in my experience, mostly older and male. Both activities can be as intense as you want to make them. Whitewater kayaking has a younger demographic. I took up whitewater canoeing at about your age. Look for clubs and lessons.
Whitewater canoeing is definitely a challenge. It might depend on where you are. I live near the mountains, so white water canoeing works well. Sea kayaking is done on lakes around here, but more serious in the ocean. I like canoe camping. I have two canoe camping whitewater trips scheduled for July. Canoeing rivers is a social activity, partly for safety and partly for the logistics of the car shuttle. Flat water canoeing is a different activity, generally less intense, more about the camping.
Some folks use inflatable kayaks on whitewater, more forgiving. Probably you would be happy in a hard shell canoe, since you like challenges. ;-)