I remember dreading 40 and my sister said that she had liked it because she enjoyed the feeling of being at the young end of the age-decade, rather than at the old end. eg 40 felt like being the new girl in the Fortysomethings club, whereas 39 felt like being the old battleaxe of the Thirstysomethings club. And, probably because she told me that I felt the same, and I am looking forward to being the new girl in the Fiftysomethings club in a couple of years time.
In my twenties I thought I was grown up but my emotions, judgement and experience were little better than when I was a teenager.
In my thirties I felt more grown up, having experienced a bit of trauma/heartbreak but TBH the first half of the decade was lost to worrying about whether I would be able to have a baby or not and the second half was lost in a fog of pregnancies, babies, anxiety and wiping up excrement.
In my forties I feel I understand myself more, have come through the upheaval of having small children, enjoy my job, love my husband, consider myself incredibly lucky. The downside is that I look so old - how did that happen? 
When I was 20 I used to sneer at celebs who had plastic surgery - why can't they just grow old gracefully???? - but now I think "ooh I'd LOVE a facelift." 
And my libido, well dunno where that is, I think it might be hiding in the cupboard under the stairs. The "revving up" of the early 40s never seemed to happen for me. Or perhaps it did, briefly, which is why I had another baby at 43 and that dampened it all down again. 