I can answer your questions. I’ve worked at Camp Simcha for nearly ten years this summer.
1) You must be 18 years old to be a staff member (lifeguard, waiter/waitress, etc) and 19 to be a counselor. Occasionally they will accept a 17 year old with good references for a mother’s helper job to assist the head staff. 2)You should apply early in the school year. The deadline to apply and interviews begin approx December so make sure to check for the deadlines and get all the paperwork in on time. You find the applications and deadline information on http://www.chailifeline.org or http://www.campsimcha.org. 3)Camp has two sessions for boys and two for girls. (Camp Simcha for children with cancer and Simcha Special for children with chronic physical conditions). Each one is approx 2 week long. Depending on the calendar and yeshiva zmanim, some summmers it is 4 week girls in a row then boys and some summer it could be 2 weeks girls, 4 weeks boys, then 2 weeks girls again. The dates for next summer will be posted on the site once the applications become available.