As of January 14, 2020, Microsoft will no longer support Windows 7
What does that mean? If you are still running Windows 7, you will not recieve updates or security updates. This will leave your computer vunerable to hackers, viruses or malware. You are free to use any version of Windows you own for as long as you like. Programs and hardware will not recieve updates after the operating system has reached end of life. Microsoft is extending support until 2023, however some users will be paying a fee for updates if they choose to keep Windows 7.
You can use your Windows 7 key to update to the appropriate version of Windows 10 if you did not take part in the free upgrade promotion. If your system can be upgraded to Windows 10, I would advise doing that. Windows 10 is like a polished version of 7.
I recommend the following specs for Windows 10:
If you still want to keep your Windows 7 and want to use Windows 10, you can purchase a new pc or build a new pc, and install a virtual machine within Windows 10 to run Windows 7 with your product key. Just note you will need more ram for virtual machines.