Goodness yes, I'm 45 and have just been to a 40th birthday party. I've been to quite a few over the last couple of years, aswell as a couple of 50th birthday parties. My parents, or indeed anyone from their generation, wouldn't have dreamt of having a party to celebrate their 40th!
I too go out regularly with the girls, we've been away a couple of times aswell (Barcelona and New York). I dye my hair and wear trendish clothes. My Mum at my age had a perm and wore a tweed skirt! The only times she went out socially were to a Church social (they used to have hotpot evenings - I bet they were a right laugh!), for a meal a couple of times a year with some of her friends (and then straight home afterwards, none of this knocking back the wine!) and to a dance a couple of times a year with my Dad.
I think that my Mums generation thought themselves older than they were. I think that our generation think of ourselves as younger people rather than old, which our parents did. I bet we have more fun!