Pretty simple: I've planted a cactus on sand with no blocks around it, and I left it alone for about half an hour. It didn't grow, and when I pressed F3 and looked at it, it said the cactus was at "age: 0"

Steps to Reproduce:
1. Got cactus (specifically, I got it from a flower pot which I found in an igloo)
2. Planted cactus on sand, with nothing adjacent to it horizontally, diagonally, or vertically.
3. Cactus won't grow.