Sophia Loren. Absolute favorite. No question.

I've never much understood why so many people go crazy over (as Dante mentions) Hepburn and Monroe; Ms. Loren still stands, stunning, at 79. More than capable as an actress, and still doing stuff today (Human Voice, for example, released this year, based on the Jean Cocteau play) and I find myself laughing as I've watched the smoky almond-eyes come back into fashion among a generation that seems to have no clue who made it popular the first time around!

Also, I keep forgetting that Annette Funicello got her start as a kid; I fell in love with her in my early teens watching the "Beach Movies" with her alongside Frankie Avalon. While they did try to pass her off as a "teenager" in the films, she was already 21 when the first one was filmed, and very obviously older than any teenagers I knew at the time! So, I guess she doesn't count for the sake of your question, but I did certainly feel more than a touch of heartbreak last year when I heard she'd passed away.

Catherine Deneuve is another actress high on my list, though, and I don't consider her career starting at 13 as "when she was a LG." The Umbrellas of Cherbourg has been one of my most-cherished films through much of my life, and then seeing her (alongside Susan Sarandon) nude and making love in The Hunger was quite the pleasant experience much later in life!