I am 28 and I have had acne for 16+ years. It has always been off-and-on. The past couple years I've been trying to identify a pattern.
During the past 5 years, my acne has been better than before. During the past 5 years, I have been eating healthier. However, I recently went through 3-4 months of bad acne again. It was primarily around my nose, and secondarily under my mouth. Those huge ones that start and build big deep under the skin, eventually surfacing to massive ones.
I observed that around the same time the worse acne started, a new donut shop opened in town, and I was eating tons of donuts. My donut intake skyrocketed since I'd stop there about twice per week, and people would bring them into work & I'd eat many of those.
So, about 2 months ago I stopped eating donuts, and my face has now been clearer than it has been in 16 years.
Anyway, I'm seeing a lot of topics/posts saying that donuts don't cause acne, so I wanted to get this observation online that from what I can tell, for me personally, donuts are pretty directly involved in the equation.
So, if you can't seem to identify the cause, and you eat donuts, try cutting them for a couple months and see what happens.