Have you personally known someone who spent a very significant portion of their life "in the closet" only to finally "come out" much later?

Yes, David De Silva, a Portuguese-Indian-Australian.

He was born gifted with extraordinary physical beauty and grace - but born into the outer western suburbs of Sydney, which is a working-class area that has traditionally had little or no tolerance for diversity in gender and orientation. He was naturally "effeminate" from earliest childhood and was constantly bullied for it - treated as if gay long before he knew his preference.
When I first met him, he was a young man of around thirty in a relationship with a woman, Betty Alexopoulos. Both were ballet dancers in the Russian tradition - best friends with deep devotion to each other - but, they confessed to me when asking for advice, having dreadful difficulties with sex. She was still a virgin because he couldn't get in. Seems extraordinary, almost unbelievable at that age - but given his background - and hers being raised as a good Greek girl in an Eastern-Orthodox family - well... I guess exceptions happen more than we realise.

Years passed. Ari and I moved 800 k's north to a rural life in 2002. We lost touch with quite a few of the people we'd previously known.

Then a few years ago David rang to say that he'd discovered he is gay, had fallen in love with a beautiful man, and was coming out.
His voice was so hesitant.
I reacted with joy.
"Oh, David! How wonderful! I'm so happy for you that you've finally found happiness."
We talked about it for a while - who the lucky man was, what he was like - how David was getting on in dancing (has massively expanded his repertoire in Classical Indian and modern dance - much involved with experimental performance - and artist working in a group - successful in getting grants.)
We ended with him and Paul having a permanent invitation to come and stay with us if ever they wanted a holiday in the country.
Haven't heard from them since. Am guessing that city life is infinitely busy - too many commitments.

Am also acquainted with several women who only came out very late. Each had known she was gay at an early age, but having grown up in a conservative Christian, Jewish or Muslim family, followed their family's expectations by marrying and having children.
Much later, each met some special woman and formed a close friendship. Over time, the attraction grew. They fell in love. It got so strong that eventually they had to come out to their families. The shit hit the fan big time. Divorce. Working out parenting rights and schedules and finances.
In each case, the family has eventually, reluctantly, accepted the reality.
They still get together at times like Christmas or wedding and funerals - but they still believe their aberrant gay relative is destined for Hell.

Jenny is a good example. If you'd known her in her former self, you'd have seen her attending the Anglican Church on Sundays with her husband in a suit and tie, her kids in their best, and she in a floral frock looking like something out of the 1950's. She looked demure, mousey and miserable.
Today she majors in Queer Theory at Southern Cross Uni and dresses like a butch dyke. Her stride bounces with vitality.

The kids are always the ones who have adapted best. They're just happy that their Mum is finally a fulfilled person. And in each case, they happen to like the new partner. I notice that the step-Mums make a big effort to make friends with the kids and not to interfere with the mother's parenting style. Maybe that's a characteristic of the values that go hand-in-hand with honouring difference.

Yes!  It's one of my cousins (mom's brother's daughter).  She's a couple of years older than me and she didn't come out of the closet until she got engaged to another woman which was three months ago so we're going to her wedding which is this October!  Yay!  A wedding!  Before that she just complained a lot about never being able to meet a guy and she talked a lot about guys but you could always tell that other girls are what did it for her so it just wasn't a surprise at all when she came out.

Anyway, the wedding's in October.  My uncle is a hardcore right-wing Christian so yeah, he doesn't approve, obviously.  It looks like we won't be going to his house this Thanksgiving for dinner or this Christmas for dinner or anyway holiday after that for that matter because anyone who goes to the wedding is getting permanently disinvited from holidays at my uncle's house.  He's really isolating himself and my aunt by doing that because that means my mom the only one who's not getting a holiday dinner ban.