Alot will depend on how history heavy the events are. If they are simply theme camping, nurses at events or women poorly dressed as men will be seen...
If you are concerend with history and authenticity, then women nursing the woulded would be foud at general hospitals very far from the battle or in private homes days after the battle. Civilians tended to flee battle areas for obvious reasons. If she wants to serve as a male, then she could serve as an orderly or litter bearer (litter bearers require strength and stamina and are not a job for those who can't work hard carrying a grown man on a litter). But she shopuld be able to really pass as a man. If she looks like a woman dressed as a man, she should think about just portraying a woman. And as the best women reenacting as men will tell you, if you dress as a man, don't even think about going to the ball as a woman. Some women can do a great job passing as a man, so if she can pass convincingly as a man and wants to she could think about it.
As for reenacting age, that sounds like a unit requirement. Shop around, other units may tell you otherwise...
Medical Director Bee's Brigade - 150th First Manassas
Medical Director Evans' Brigade - 150th Leesburg
Medical Director Valley District - 150th McDowell
Chief Surgeon of Division - 150th Seven Pines/Seven Days
Chief Surgeon of Division - 150th Sharpsburg
Chief Surgeon Heth's Division - 150th Gettysburg