I'm in my 20's and find it very easy to get an erection, often I will wake up with one and I can also maintain an erection for hours if i am just thinking about it while i am lying in bed.
My problem is sustaining a "hard" erection during masturbation or sex. After a while of masturbating my erection tends to taper off a bit and become less hard (not to the extend that it becomes flacid - just weaker). Then when i stop for a couple of seconds, my erections recovers somewhat and gets more solid again.
It's really annoying b/c at other times the hardness is a lot more when i am not doing anything!
Could this be a circulation thing, as my hands and feet often get cold in winter and i sense my heart sometimes has slight pulpitations.