Since the 6s, iPhones get 6 or 7 versions of iOS (including their release version), which puts them at about 7-8 years of full support less whatever features (like Apple Intelligence) don't get support on some hardware. Before the 6s, it was lower but the hardware was also changing a lot faster (like going from 32-bit to 64-bit ARM).

But for security updates they've pushed it up to 10 years. The 6s, 2015, got a security update this month.

They're actively supporting (at least security updates) iOS 15, 16, 18, and now of course 26. 17 was skipped because no devices lost support with it, everything that could run 17 can run 18.