I'm 47 now. I think when I was under 18 the general idea of middle age was sensible clothes, grey hair (women's always short), definitely no clubbing or watching rock bands live, probably having teenage kids, owning a house. I knew the sensible clothes things was't necessarily true, as my mum always had interesting and unconventional hair and clothes.
I think the going out thing has changed a lot as people have spent more time doing things like clubbing and going to gigs before they 'settled down', so they become a big part of your life, they're not just a thing you do for a bit while you're young and then stop. Also more people being single and not having kids and you're more likely to have friends who never stopped going out. Not that it's a massive part of our lives, and certainly dialed right down during our 30s when kids were younger, but we're rediscovering going out a bit more now our kids and our friends' are older and we're not just doing 'grown up' things.
It's funny how our generation is still wearing stuff that signifies the pop culture of our youth - at a friends' party at the weekend and spotted a Hitchhikers Guide to the Galaxy bag and a Ghostbusters hoodie (and funnily enough, someone's teen in a Ghostbusters t-shirt). You can't imagine our parents going around in their 40s in Lone Range t-shirts and Muffin the Mule bags or something.
We do own a house, but we are pretty lucky, especially in terms of not having a mortgage any more. My mum had 3 teens by the time she was 40, I was at the stage of having two teens at 46, and that's relatively young by my peers' standards.