I don't have a problem with the age gap. In fact, when I was nineteen I had a relationship with a man eight years older than myself. My daughter has been seeing a man with an even bigger age gap between them for some years. I've never been bothered by it, but the difference is we were both adults when the relationships started.
I like Tyler (well I used to anyway) but I think he has very little in common with Piper and his judgment has proved to be questionable at best. I doubt their relationship would last - at least not in the real world.
Saying all that, Therese and Brad are in a very tricky position because banning them from seeing each other is likely to exacerbate the problem. I think I'd just ban them from sleeping together until she's seventeen if I was them. Of course it's easy to be objective when you're not emotionally involved!