As someone who has a past life incubus I think it depends, and it depends on the person, the circumstances of their life, and how rapidly they mature both physically and mentally. Personally, I found out about Him when I was 16yo, 3 years after dedication, and back then He wasn't behaving like a lover, even if He clearly seemed in love with me judging by the energy. I once showed Him clearly that I wanted to have sex right then, I was about 16-17, and He took a few steps back, I could slightly see Him like a blue light silhouette in the dark, getting further. Gradually, He started to behave with me differently, and show me by act how attracted He is.
We still can't touch properly because the huge bioeletricity difference. I'm yet extremely young, 19, but the way He approaches me changed gradually as I said, as just a few weeks ago I heard Him while changing clothes saying to not dress back, He was laying in my bed looking at me so I layed almost naked next to Him, and He started touching my body (or at least trying to, as His touch still burns and sometimes gets me trembling), and kissing me, again, trying to, but it was feeling really powerful, so I closed my eyes and when I opened them I could see Him the clearer I have ever seen Him, like you'd see a hologram but extremely light. He smiled at me and got his face close to my naked chest and I could see his curly blond hair and deeply in love light blue eyes.
I wanted to point out the difference I felt between these two phases of growing tho, judging by the things I felt from Him, and that is, in the first one, I felt He took those steps back because my body wasn't mature enough and I felt it right then, while now the only problem is just that He can't touch me, and even like so, He comes as often as He can just to touch me, as close as He can come yet. So... when you're a teenager, your body still needs to grow up, and it's a whole hormonal process going on, even if you're maybe not growing in height, while at 18-19 this process of "growing up" usually stops, even if you're still young and going through changes.. everyone goes through physical changes tho, at no matter what age.
Now coming back to what I said first when I started writing this message, "would you have to be old enough to have a demon lover?" No. I mean, I had mine when I was even 5 yo, even if I wasn't conscious about it lol. The age depends when it comes to sexual or more personal contact tho. That's usually after you've grown up, and that can be as in my case 18, it can be bellow, like 17 but I don't think it goes lower than that or it does in extremely rare cases and it can be later like up to 20-21 maybe.
But just as Shadowcat said, despite the fact that you might have just grown up physically, this doesn't automatically give you the mentality necessary for having a demon/ess lover, as usually people until their late or mid twenties barely know their own selves and their needs when it comes to relationships and not only so there's also a chance a demon/ess won't want to have a relationship with you, because you're unstable. However, everyone is different, and to talk about what NinRick said a little, for a god it would be barely different if they had some lover of 20yo or of 30yo as for them these are both very young, so a 20yo can be unstable while a 30yo can be a little less unstable, ya know? There's not much difference in their eyes. Let's not forget the little we know about how gods perceive liniar time and try to understand this a little. Also, everyone is growing differently, so someone at 20yo can be far more mature mentally than someone in their 30s depending on how they've both grown and evolved during the years. And also Gods can see the past and can see the potential futures, so they know whom they're fucking with.
When it comes to these things we can barely generalise situations, as we should mostly be talking about satanists. Even like so, there are some deluded teenagers watching porn and believing they'll get, if they ask for it, a some sort of entity which will satisfy their unrealistic and toxic sexual needs whenever and however they want or whatever kind of deluded or rather uninformed people and these kinds pop up like mushrooms talking about their experiences in their dreams or what their thoughts told them and so on. Again, generalising doesn't help at all and this can be a very long discussion if we were to think about the most common possibilities, as there are lots of kinds of people anyways.
So to put it more in short, you can have or even ask for a lover at any age, even if they won't behave like in an actual relationship, so you won't have sexual contact until you're able to, and that's when you've grown up physically, and you have to be mature enough both mentally and physically in case you get one when you ask, because Satan can refuse this.
Personally, I didn't understand why a demon would choose to be with someone so young either, but when I asked my lover this about us a few times I got some short personal answers I'm not gonna talk about here, but for me it's understandable enough, at least in my case. So.. this is all what I think, even if I didn't say all that I initially thought of in this whole message, I suppose that as unexperienced as I may be I can have an opinion about this, as it includes me in a way or another, so I felt I had to get into this conversation now for some reason if that's okay.
