How the charity was transformed into a source of innovative grants in northwest London, now running at £5m a year, is told in a sumptuous new book, A Long and Winding Road by Nick Owen, published to celebrate the charity's 21st birthday this year.
It may seem ironic that such distinguished beneficiaries of the JLC - the Young Writers Programme at the Royal Court, the Unicorn Theatre for children, the educational department of the Donmar Warehouse, the Barnet artsdepot and the amazing Shakespeare project that has been rolled out from a primary school in Camden Town - should hark back to the founding of a bastion of privilege, Harrow School, in 1572.
That came about because of the granting of a charter by Queen Elizabeth I to John Lyon, a yeoman farmer, whose purchase of land to maintain the two great roads has resulted in a bequest to the Governors that has since funded both Harrow itself and the John Lyon's grammar school until, in 1981, a decision was taken to try and convert the charity into a youth education fund.
With the careful management of incomes from the land - including a large part of residential St John's Wood - the charity's assets stand at something like £250m, an astonishing yield in these straitened times, and the newly formed JLC has, to date, distributed over £72m to charitable causes.
The book - which may be purchased through the charity at 45 Cadogan Gardens, London SW3 2TB (firstname.lastname@example.org), and to which I have contributed, but don't let that put you off - tells all sorts of stories; it's part history, part social document, part evidence of how the arts work for and with young, mostly disadvantaged, people today, and a fascinating summary of the economics of leasehold reform, funding and investment, with a plentiful supply of charts, maps and tables.
The charity threw a party last night in Kensington Palace attended by arts and projects leaders, representatives of family support groups, sports enterprises, law firms and bankers, and the staff of JLC, led by the clerk to the governors, Andrew Stebbings, and the grants director, Cathryn Pender.
Pender succeeded my friend David Robins, who died in 2007, as grants director and, although she had a very hard act to follow - Robins more or less defined the objectives and peramaters of the new charity in 1991, and was generally regarded as the JLC's "heartbeat" - she has done the most fantastic job of administering the grants (all of which have to be approved by committee) with as much flair as discretion.
Writing my reports and visiting the work in progress has been one of the most enjoyable and rewarding parts of my own life for some years now, as it has been for such other advisers as visual arts specialist Teresa Gleadowe and musician and conductor Martin Neary. And the party was a very agreeable excuse to huddle together and swap notes for a couple of hours, helped along with champagne and canapes.
More agreeable, certainly, than huddling under blankets and winter coats in Regent's Park the night before at the opening of Timothy Sheader's terrific production of To Kill A Mockingbird. Will summer never come? Or is freezing in the cold of outdoor summer theatre just another cruel torture we must bear in hard times?
We're all hoping for sunshine at some point in this coming Bank Holiday weekend, and I hope some of it breaks through this afternoon as I'm on my way to Stratford-upon-Avon to see Titus Andronicus, whose last act is surely the ultimate episode in the Great British Bake-off... but the forecast is poor and the dismal tragedy will be suitably shrouded in rain and dark clouds.
But at least we'll be inside, in the Swan. And I see that the RSC restaurant is serving a special Titus game pie and a Queen Tamora bloody cocktail. I might just pop elsewhere for my tea.