Or a more realistic answer is you can use what you are happy with. If you are on seven and happy, end of. Just no more updates and if you buy a new computer, you MAY just have to accept ten unless you can find one installed pre ten - which is likely.
I recently changed my PC and now have Windows 10. It was a struggle at first, having to learn new things, but I seem to have managed OK, mostly due to help on AB !
Its not as bad as it seems, although I would go back to XP if I could !