I think most interns are in the 18 to 25 year old range but I guess anything over 18 (or whatever is legal to work in your state would be fine)

As to what to put into your resume. Put down what you have done and write a good cover letter. GPA is not really relevant unless you are a strait "A" Student and in the NHS and sports activities are not really relevant unless they directly apply to the job you are applying for (an audio engineer for play by play football and even then it is really not relevant). If think if you show you have a lot of jobs in a short period of time the prospective employer will be some what questioning as to why you took and left a lot of jobs but at a young age today that may not make so much of a difference. Tell the prospective employer in the cover letter about what your goals and aspirations are for the job you are seeking with them and what you hope to do with the knowledge you will acquire. Tell them about your work habits and how this job means much to you but don't get overly pushy. The best way to get a job is to know a lot about the company before you apply and to tell them about how you would like to help them do whatever it is that they do well. Blanket cover letters that are sketchy on details are IMHO a sure turn off. It simply means the letter writer did not want to take the time to write a focused letter aimed at getting a job at a particular company but did an one-size-fits-all letter and simply copied it on on copier. Be honest with all your accomplishments. Don't include references but say "a list of references can be provided" and when you give them the list, if they request it, make sure all the people on the list have said OK to using their names and you are pretty sure they are going to say what you want them to say. Never put into a resume something that you did not do or "blowup" some job that you did like saying you were the distribution manager of a newspaper but in reality you were a paper carrier.

Best of luck! There are lots of suggestions on the WWW and you may want to do search of "how to's" when it comes to writing cover letters and resumes.