Mint used to stick to the latest Ubuntu till Linux mint 16. Starting from version 17 they only track the long term support versions of Ubuntu. Mint 17 - 17.3 were based on Ubuntu 14.04. Mint 18-18.3 will be based on Ubuntu 16.04.

They do have another version that is based on Debian directly, but I have never used it so can't say much about its upgrade paths.