Sleepgasms is my preferred term 
Apparently wet dreams or sleep orgasms do tend to decrease as you get older as your hormones are more settled, but they do still happen. They’re completely involuntary though and I don’t think there’s a huge link between how much sex you’re having/not having. However, if you’re sexually active then you might be more likely to have sex dreams which can lead to them 
There is some research that says sleeping on your stomach is more likely to lead to sleep orgasms but I don’t know how proven this is 
There’s some more Sleepgasm chat in this thread too 