Interesting. While I think giving money to political campaigns
  is generally kinda ridiculous for ANYBODY to do, nevertheless if
  you want your voice heard in some way, you should be allowed.
  Anyway, an interesting side of things I hadn't thought of. It's
  a form of [1]#ageism - theoretically designed to prevent abuse,
  but there's no law in the USA that prevents people under 18 yrs
  old from donating to political causes and getting their name on
  the rosters as supporters, so it's really the choice of the
  candidates ultimately.
  http://youthrights.org/2016/01/19/the-nine-presidential-candidates-who-dont-want-your-support/

References

  Visible links
  1. https://plus.google.com/s/%23ageism