At least 18 to donate plasma. You won't be allowed to donate if you are taking Accutane, or have had a tattoo within the last 12 months.
Other conditions can keep you from donating too.
Semen donation sounds like an easy way to make money, but it's actually not very easy. After going through a physical, your semen is frozen for six month, and then you are required to make another donation as a followup, to ensure that you have no STDs. Not until then is the first batch used.
And you have to be a real stud. I seem to remember reading somewhere that only 3-5 percent of all semen donors are actually used, because they have to produce prodigious amounts of live semen
.
This is a rather entertaining account of someone who wanted to know what was involved in becoming a semen donor:
http://www.slate.com/id/2119998/
East Hill