One word: Tubbs.
Pre-12 I think it was probably the middle daughter on Petticoat Junction, Samantha on Bewitched, Marlo Thomas on That Girl. I think I was older than 12 when The Partridge Family was on but I had a huge crush on Susan Dey.
Just remembered another from when I was around 12, Lara Parker, who played Angelique on Dark Shadows.
He turned into kind of a chinless wonder as he grew up; I always thought of him as Plumb-Bob.
I wasn’t super into girls before 12 but I remember thinking Sam (Alyssa Milano) from ‘Who’s the Boss’ was pretty interesting. The cast of ‘Saved by the Bell’ got my attention too, although I might have been older at that point.
I actually dated someone who looked like Jessie when I was 18 so it must have stuck with me.
Sally Field on Gidget when I was ten. 
Louisa Milwood-Haigh.
Who? She is an actress who starred in a sci-fi show I watched when I was about sixteen, called The Gemini Factor. A good show, (which I missed an episode of, annoyingly) and she was very cute. She hasn’t done much else since then, just small roles in various different UK series scattered here and there, but from what I can tell, even though she’s now in her late 40s, she still looks exactly like my type.
Both James Franciscus and Bruce Lee from Longstreet.
Cat Stevens
Pretty much all the Monkees
Lee Majors for The Big Valley
Michael Landon from Bonanza
Gene Kelly and Fred Astaire
Randolph Mantooth from Emergency!
StG
Sheena, Queen of the Jungle., on late 50s TV. Tarzan’s female equivalent. Definitely a swinger.
I remember watching Logan’s Run when I was 10 years old. Fell in love with Jenny Agutter.
my brother was 3 or 4 years old when vanna white debut on wheel of fortune and was fascinated with her to the point he purloined a special compilation book of her playboy pics from a neighbors house when he was 11 or 12
This reminds me of David Selby who played Quentin on Dark Shadows - I had a crush on him too.
My little sister had a crush on Barnabas!
Very young, Agent 99 on Get Smart and Ginger on Gilligan’s Island. When I was older I became a Mary Ann fan.
When I was around 10 or 12, Daisy Duke from Dukes of Hazzard, and Wilma Deering and Queen Amidala from Buck Rogers.
As a teen, one semi-obscure crush was Carol on Growing Pains. Another was Emma Samms.
I’ve read that Hoffs wasn’t aware that there was a closeup of her while they were filming the video. And the reason for the side-long glances is, because of stage fright, she would focus on individual members of the audiance. Although perhaps she was looking for you, and was disappointed when she couldn’t find you…
Any and all of Alexander Mundy’s (Robert Wagner) girlfriends on It Takes A Thief.
I don’t if anyone has done gay yet. But before age 12, I would look forward to seeing Gil Gerard as Buck Rogers and Robert Wagoner as Jonathan Hart in Hart to Hart. I should point out, it wasn’t so much a crush. I just thought they looked so handsome. And I felt so awkward at that age.
I hadn’t even discovered I was gay yet. But yeah, it might’ve had a sexual component too.
That would be Princess Ardala
Queen Amidala was Star Wars
When I was really young it was Helen Crump on the Andy Griffith Show.
Emma Samms, mmmmmmmmmmm!
The first that comes to mind is Meredith Baxter from her first show “Bridget Loves Bernie”, not so much from “Family Ties”. “Bridget Loves Bernie” played right after “All in the Family”, so it had a prime spot. I don’t recall much of the actual show (I think it was kind of mediocre). But I’d be transfixed just watching Meredith.
Also had a huge crush on Susan St. James from “The Name of the Game”, “McMillan and Wife”, and later “Kate and Allie”. As I look back, she was kind of like a prototype Sandra Bullock.