I have a friend who's never been married and never plans on it, nor does he have any children (and also never plans on it). He turns 40 next year and plans on having a big formal party to celebrate it - much the same as you would with a wedding reception. He even plans on registering for gifts, since he'll never have an actual wedding reception. (It makes me jealous because I didn't actually think to register for my 30th!)
I think it's an incredible idea and I hope it works out for him. However, he's a spurious, capricious, contrary little man, and I think it would be an absolute riot if he ended up falling in love and marrying someone after the party but before his 42nd birthday. Knowing him, I could easily see it happening.
And no one grew into anything new, we just became the worst of what we were."