Jesus. Hes obviously watching HER, not randomly looking out the window. I love looking out my kitchen window, a fair number of people come and go (it's a bit of a thoroughfare and near a town) and I frankly like to see who's about (kinda from a security point of view).
The front bedroom where my DD sleeps faces directly onto some flats so she has venetian blinds which she keeps closed so no one can perv on her. I'll glance out of those windows when I'm up there clearing her floordrobe.
But I can say that I NEVER stand at the window in the ight or in the dark, staring into any of the windows opposite. Additionally, if I was looking across and happened to see someone sat on their own I would go out of my WAY to make sure I wasn't looking at them or give them any cause to think that I was looking at them - because that's the last thing I'd want.
Like OP I've been looking out of my window - for my grocery home delivery - and caught the eye of a neighbour and though oh God I bet they think I'm looking at them to see what they're doing! I'm not. So what do I do? I desperately try to avoid doing that because it would freak anyone out if they were gawping at me.
Surely it's obvious it's totally intentional and perving in nature, or at the very least incredibly incredibly incredibly nosey and intrusive, But likely pervey, sorry, all the facts point to it from OPs description.
Also, I like to be able to see out my windows, I don't like curtains or blinds or anything which restricts my view and wouldn't be happy if I had to have them at the window because someone was openly gawping at me a lot of the time.
Whether he's 50 or 90, he should (if he had half a brain) be thinking about how his actions (which are not someone just looking out the windows casually) could be interpreted, could they cause concern or distress to the person being observed, is it an unreasonable amount of gawping that he's doing preventing OPs reasonable enjoyment of her own home fgs. The answer would be yes, it's not reasonable and there is a pervy edge to it that is completely inappropriate and understandably unsettling.
I'd say f the curtains/blinds/nets (shudder) and the next time you see him go over there (it'd be ideal if you had a witness to go with you) and say what many PPs have suggested asking him why he keeps starting at you you find it unsettling and inappropriate that he's watching you with the lights on and off and you would like him to stop. That seems entirely reasonable to me.