The only problem I could imagine with being 40 is a certain attitude; if you come to interviews too confident and give the impression you're there to teach ever

The only problem I could imagine with being 40 is a certain attitude; if you come to interviews too confident and give the impression you're there to teach everyone and show them what's what, it could be off putting; even if you ARE more experienced and are hired as a senior, the people hiring you want to know you're gonna be a good sport and not MR know it all. Or it could be something else completely. But I would definitely explore attitude some more.