Age limit to donating blood

I noticed that there are normally age limits to donating blood, 17 being the most common. Why must there be an age limit? Isn't a 16-year-old's blood just as good as a 17-year-old's? I can't imagine it would be that bad for a person's own health to donate blood. Is it because of responsibility? Like a kid might try and uh, donate more blood than he should? Seems like people normally care about themselves and don't donate when almost anybody could.

Most states consider those below 17 minors and since blood donation is not without risk, would require a lot of paperwork etc. to have a teen donate.

In-Depth Discussion of Age and Blood Donation

Those younger than age 17 are almost always legal minors (not yet of the age of majority) who cannot give consent by themselves to donate blood. (Each state determines its own age of majority, which can be different for different activities.)

Persons under the age of 17 may, however, donate blood for their own use, in advance of scheduled surgery or in situations where their blood has special medical value for a particular patient such as a family member.