Back when the fake Hitler diaries were big news in the early Eighties, I remember The New Yorker had a cartoon showing a bunch of stuffy academics reading aloud from them. The caption was, “Dear Diary: When I have conquered America, I must meet Betty Grable. What a tomato!” 
As a kid: Leelee Sobieski from Jungle to Jungle (holy crap she’s nearly 30 now… I feel old).
Teen/adult: Rebecca Hall from The Prestige and The Town (holy crap she’s the same age as Leelee! Funny coincidence).
I’ll definitely agree with you on Barrowman and Tennant. And the accents. Is there anything sexier than Tennant talking??? Add Christopher Eccleston to the mix and that takes care of adult for me. 
Kid: David Hedison and David McCallum
Teen: Chad Everett, David Soul, James Brolin
Kid: David Yost (the blue ranger in The Power Rangers), and Jonathan Taylor Thomas. I think both have come out recently, which amuses the hell out of me for some reason.
Teenager: Orlando Bloom, Elijah Wood…hell, half the male cast of Lord of the Rings. Also, Sean Biggerstaff, the dude who played Oliver Wood in the Harry Potter movies.
Adult: Zachary Quinto (“That guy who was Spock in the last Star Trek movie,” dangermom :D), Christian Bale, Paul Bettany, Hugh Jackman, and Adrien Brody.
As a kid: The girl who played DJ on Full House. Julia Roberts.
As a teen: Pretty much any girl from Saved by the Bell, but especially the one that played Kelly. Julia Roberts
As an adult: Eliza Dushku. Anne Hathaway. Julie Andrews. (I don’t care if she is 27 years older than me. She’s hot.) Julia Roberts. Jamie Lee Curtis.
Kid: Valerie Bertinelli and Maryann on Gilligan’s Island.
Teen: Diana Ross (THAT album) and Rachel Ward (The Thornbirds)
Now: Don’t really have any, it’s transient. Jessica Alba was hot in Dark Angel. I guess the most recent was Morena Baccarin from Firefly.
As a kid: Michael Jackson.
As a teen I was more realistic and knew celebs were out of my reach so it was just the boys in school.
As an adult: Yun-Fat Chow, Bono, and a host of other bits of eye-candy.
As I kid - I can’t remember. So I’ll go for the earliest Crush I can remember - Alicia Silverstone (I am guessing I was somewhere around 13 when I had that crush, but I could be way off)
As a Teen - I had LOTS of crushes so I’ll pick one of the more memorable ones - Shannon Miller (US Gymnast)
As an Adult - Again, more than one. So I’ll plump for Scarlett Johansson.
Those Activia commercials will give anyone the hots…
As a kid, my first “celebrity” crushes were around age 5, on Bernadette and Edith, two of the girls on “Zoom” (Anyone remember “Zoom”?).
As a teenager, I had a crush on Liz and Jean Sagal, the twin actresses who starred in the mercifully short-lived sitcom “Double Trouble.”
As an adult? Maggie Cheung in “Irma Vep.” Rowr.