As far as I'm aware, first aid training certificates last for three years, or at least the one I have which I did the course for earlier this year does anyway. This is worth noting if you have done a first aid course a few years ago, as there have been some changes to recommended procedures for CPR and AR on adults, but more particularly when doing so with youngsters (this I know because the course I did included covering Pediatric First Aid).
Coincidentally, I actually used my First Aid last week at work, when someone I was training exhibited three of the main symptoms of a stroke, although of course the main thing I had to do was to simply call an ambulance and let them get her to hospital after I done the tests to confirm my suspicions. It's worth noting here too, that the girl in question was only 24, so don't imagine that you have to be old to have a stroke, and it's worth being able to spot one, because the sooner you get someone to a hospital if they've had a stroke, the less likely it will be that they'll suffer pernament injury.
If you are likely to be at an airsoft site where an ambulance might not get there easily or quickly, I'd recommend a few other things for a medic bag, such as some sort of clamps or forceps (useful for pulling bits out of a wound amongst other things), some small scissors (good for cutting bandages, cutting away clothing to get at an injury, etc), some eyewash (for obvious reasons), a space blanket (to keep someone warm if they are suffering from shock, which is likely with severe bleeding, and very dangerous if not treated). There is also the possibility that you might consider an epi-pen if you are a first aider at a woodland site, although it should be noted that these are not all the same, and specific training is required to know when and when not to use one of these. Nevertheless, it is possible that someone could get an adverse reaction to a bee or wasp sting (particularly when combined with running around) and suffer anaphylaxis, and this can easily be fatal if not treated.
You might even go 'all Vietnam' and throw in some cyanoacrylate superglue (which as you probably know, was tested successfully in the Vietnam War for closing wounds, in order to slow down bleeding, and this was before it was actually medically approved for that use, although it did work and apparently saved quite a few lives). A word of caution here though, actual medically approved 'superglue' (branded as Dermabond if you buy it) uses a 2-octyl cyanoacrylate formula, whereas the kind of superglue you get from a DIY store uses methyl-2-cyanoacrylate. This is because the latter can cause skin irritation and it also generates considerable heat as it goes off, although this is only likely to cause a burn if you use a lot of it. Nevertheless, for a really bad injury where you have to stop bleeding if an ambulance is a long way off, you never know, it might actually be useful to have some to hand, after all, you don't have to use it, but you can't use it if you don't have it.
