The target audience is pretty wide, to be honest. Mostly teens, young adults and adults, I'd say, or pretty much anyone with interest in space, physics, technology, large-scale politics, etc. It's trivial literature, but it has enough complexity and while early teens could read it, I think a lot of stuff would fly well over their heads. Interestingly, the average reader of Perry Rhodan is over 45 years old, according to a survey a few years ago - I think the main reason for it are those readers who got into it in their youth, and continued reading for decades.
As for this format of literature in general, there's something for pretty much any age group. Detectives, spies, agents, western stories of cowboys and Indians, scifi and space opera, adventure, ghost stories, horror, love stories, romance, erotica, Heimat novels, and so on. It surely isn't intellectual or high quality literature, but it can be great entertainment. I personally used to read them mostly when travelling, as the format is very suitable for that, and it helps to pass the time in flight or on train while not having to think too much, so also a good option if you're tired or sleepy