I completed the whole thing last year; the problems start out very easy, but get dramatically more challenging over the course of the month. Check out the statistics for how many people completed each problem in 2018: https://adventofcode.com/2018/stats

The first few problems are approachable by pretty much anyone who understands how to do arithmetic, file I/O and basic flow control. Many of the later problems require advanced skills such as more sophisticated algorithms (graph theory, dynamic programming, etc.), reverse engineering, and careful debugging.

For what it's worth, I would expect anyone who knows what a loop is to be able to get the first half of today's problem without much difficulty. The second half is a bit less trivial but also pretty easy to understand.