Most teams, fields, events are 18 plus or 16 plus. Any legitimate business with insurance may be 12 plus, due to insurance purposes. There may be a group or field in your area that is the exception, but who knows.
Gear to start:
Must haves
- good quality ballistic or paintball eyewear
- good quality footwear with at least 6" ankle support
- good quality gun $200 - $2000 (may have to upgrade it)
- batteries, charger, BBs, magazines
- headwear
Recommended
- BDUs in modern disruptive camo
- a vest or pc to carry your magazines
- a hydration system
- light, full coverage gloves
For youth
- full sealed goggles
- metal mesh mask (or paintball mask)
- helmet
- neck protector
This should maximize fun and safety for everyone. Look for good deals and but what's comfy. There are some good airsoft level gear, but avoid repros and replicas, either too expensive or made poorly. Buck up for real steel if you want your gear to last a decade. The little ones will grow out of their stuff quickly, so it isn't as big of a deal there.
I'd recommend M4 airsoft guns, they have the most parts, upgrades, gun docs, etc, etc, etc, available. Companies like King Arms and G&G make affordable starter guns, like between $200 - $300. Get you and your kids the same type of batteries, so you can all use the same charger. Cheap quality rounds will ruin airsoft guns and your experience, stick with BB Bastard or something like that. Luckily airsoft rounds are insanely cheaper than paintball rounds.
Get a sling for each gun so they don't fly out of your hands, or end up in the creek when you trip, slip, or fall.
Sniper rifles in airsoft:
http://www.airsoftcanada.com/showthread.php?t=158804