You can't take 2 cases of rude individuals to say that charity shop managers are always uptight, menopausal bitches.
You've got to remember a few things when it comes to these kinds of places, and especially in terms of the people who deal with vacancy applications. A lot of people approach such staff wanting to work specific days of the week, when there's not really a business need for it, and they'd just be getting in the way. You might be surprised to know that charities get approached by individuals wanting paid work, which is only usually found towards the higher end of the managerial scale, due to the amount of time and dedication needed and the time taken away from holding down a full-time job. Charity workers in general also have to deal with people stealing from them because they often spend less money on security, so that their funds appear well-spent. The staff themselves are also very likely to be volunteers, and if nobody else wants the job, and they're often out-of-sight, then they're also out-of-mind, and they're keeping a money-sourcing shop open and running.
I do appreciate that you've encountered a couple of rude members of staff, but perhaps you'd be better off lodging a complaint with the charity at a higher level to alert them to such behaviour. In the mean time, try other places, and try not to go in expecting similar experiences. Your help will be appreciated somewhere, I'm sure. In particular, I'd suggest local youth organisations like Scouting, Girlguiding, cadets and so forth, especially given that you're 16 and can probably be a helper in some sections.