I use windows 7 because I don't like my computer randomly restarting for hours to do updates and I prefer the original start menu. It's still supported by Microsoft and I still get updates. There is literally nothing that I haven't been able to do with it, using Visual Studio, SQL Server, Eclipse, and Android Studio.
Yes, I'm sure there are some things that only 10 will be able to do, but I've never needed them. And yes 7 is old, but because it's old it's much more tested and stable. I know eventually I'll need to upgrade, but as of now I'm very happy with windows 7.
That being said, you should use windows 10, so you don't have to upgrade in the future. My laptop is over 2 years old and I will be looking for a new laptop by the time 7 is no longer supported. Also, as it was earlier mentioned, your hardware drivers either won't be supported or not nearly as much as they would be on 10. I've had a lot of trouble getting my network card to work well on my 2 year old laptop, and would probably be a lot worse if you try finding drivers for a brand new laptop.