Airsoft is a hard thing to make a living out of at any age, let alone 14.
I'd love to work in an airsoft shop, but obviously there has to be one near by, there has to be a vacancy, blah blah blah. There aren't really any other proper jobs to be had from airsoft.
Site marshals have to be over 18, as do people in the shops for that matter, by the nature of over 18 sales being the predominant thing they do. But yeah, marshalling is only something you can do on weekends and weekends are when you want to have the time free to play! On the plus side, a lot of sites around here pay £50 a day and give you a free game day as and when you want. So marshalling actually acts as a pretty good way to fund airsoft, 'cos you can play for free, and spend the £50 on guns and shit. But yeah, 18+ only... Ignore me lol.
You could do tech work for people, but I doubt you know enough to do that at this point, and honestly even though I'm more than competent, I don't do it even though I could because I don't have the financial backing to get me out of any issues if I accidentally screw up.
So yeah... Paper round, as above. Got to be something like that you can find to do, go ask in the local newsagents or something. I used to deliver magazines from about 13 to around 19. 500 a month for just shy of £40.