There is nothing particularly good (or bad) about being forty, or not that I remember anyway(it was a long time ago). It's just an arbitary age, after all.
cynic-al has it right IMHO in that it's all about being comfortable with yourself and doing what you genuinely want to do – in other words not acting like a 16 year old arse just to appear "younger" when you really know better nor going to "out of town pubs" because you think it's what people your age "should" do (is it ?).
I'm still doing some of the same things that I did forty years ago (never mind when I was forty) like playing in bands, riding bikes off-road (without engines now though), enjoying the "company" of attractive women (or the one that I'm now married to, at least),having a beer or two, taking some photographs, whatever I fancy really.
I'm pretty comfortable with myself as I approach 58, and no, I don't mean smug and self satisfied – I just mean that I don't go out of my way to try and live up to some stereotypical image that people "my age" are supposed to have.
I wear what I like, drink where I like and mix with people whose company I enjoy. Whether they're 25 or 95 makes no difference to me.