I strongly suspect that the first time that I heard Hoagy Carmichael's music it was either sung by my grandfather, or was coming from my grandfather's record player. Hoagy's songs remind me so much of the sort of songs he loved that I can't imagine him not loving them. But I can't say that I positively recall experiencing any particular Carmichael song with my grandfather.
The first absolute memory that I have of Hoagy Carmichael is of seeing and hearing him in the wonderful To Have and Have Not, which I believe that I first saw in my early teens. I went through a Bogart phase, and I could never understand why Casablanca was more celebrated than this film. I like Casablanca, but Bogart and Bacall—plus Carmichael—how could anything top that? "How Little We Know", from To Have and Have Not, is still one of my favorite Carmichael tracks, and there is no performance of it that I love more than Hoagy casually and quietly playing it on the piano while having a chat. (As much as I think that my grandfather must have liked Carmichael's music, I also must note that when I saw Hoagy in this film, he reminded me of my grandfather. So, one way or another Hoagy is inextricably bound to my grandfather in my mind.)
Another specific memory that I have of Carmichael from my teens is George Harrison's covers of "Baltimore Oriole" and "Hong Kong Blues" on the same LP. I loved the album (though I gather that it is not a universal favorite) and these were two of my favorite tracks. Why did Harrison include two songs by the quintessentially American songsmith Carmichael on his album, Somewhere in England? I have no idea, but it worked for me.
Right now I'm listening to Disk "D" of a remarkable four disk collection, Hoagy Carmichael: The First of the Singer Songwriters, Key Cuts 1924 - 1946. It contains a terrific assortment of recordings of performances by Hoagy and others (including Duke Ellington, Louis Armstrong, Anita Boyer, Paul Whiteman, Ethel Waters, and many more). One of my fondest discoveries on this collection was the fact that Hoagy wrote music for children, as whimsically represented on this disk by his own performance of "The Whale Song". I'd have considered this purchase worth it just for that song, but happily I love the rest of it, too.
Can I imagine trying to introduce some of my younger acquaintances to Hoagy? Probably not. Some of these tracks are 90 years old, and certainly sound (exquisitely) dated, sometimes (charmingly) hokey, and at times not particularly PC by today's standards (though never mean). I'm sure that there are some younger folks out there these days who could appreciate these going-on-a-century old tracks as much as I loved them when they were already half-a-century-or-more old, I just don't know if I'm acquainted with any of them.
Of course, there will always be covers of some of his better known standards—"Star Dust", "Georgia On My Mind", etc.—which will capture new hearts. For me, though, no one will ever do Hoagy better than Hoagy, himself.