The question came up from a discussion of Neil Patrick Harris. As a child he was Doogie Houser MD, now she shines in *How I Met Your Mother with his adult portrayal of Barney.
Who else has done this?
Ron Howard in ‘The Andy Griffith Show’ and ‘Happy Days’
Mickey Dolenz of The Monkees was the star of a show called “Circus Boy” in the 50s and then one of the stars of “The Monkees.”
Soleil Moon Frye was Punky Brewster and then played Roxie on Sabrina the Teenage Witch for 3 years.
Alyssa Milano for Who’s the Boss and then Charmed.
Rick (Ricky) Schroder, of “Silver Spoons,” has done rather well as an adult, in series such as “NYPD Blue” and “24.”
Ricky Schoeder on Silver Spoons and NYPD Blue.
Jason Bateman was in Little House on the Prairie and Silver Spoons as a kid, Arrested Development as an adult. Alfonso Ribiero was also in Silver Spoons as a kid, and then Fresh Prince of Bel Air–he was about 25 when that ended. And then there’s Michael J. Fox, although he wasn’t all that young in Family Ties.
Valerie Bertinelli, child and adult on One Day at a Time
Sarah Michelle Geller was a child in High Stakes and an adult in Buffy
Anthony Michael Hall was a teenager on SNL and an adult in The Dead Zone
Bill Mumy, Lost in Space, then Babylon 5.
Valerie Bertinelli, child on One Day At A Time and adult on Touched By An Angel.
Michael J. Fox was 21 when Family Ties premiered, so he really doesn’t make the list.
ETA: Cool Ricky/Rick Schoeder simulpost.
Wow, was he that old? I was thinking late teens.
Bill Mumy was a good call. He was so great in Babylon 5.
To make up for Michael J. Fox, there was Sarah Jessica Parker in Square Pegs and Sex in the City.
Tiffani Amber Theissen. She was in Saved by the Bell and later did several series as an adult.
Susan Dey? She was a teenager on The Partridge Family before going on to be in LA Law.
Sarah Chalke and Katherine Heigl were also teenager/adult actresses (Roseanne/Scrubs and Roswell/Grey’s Anatomy)
Mickey Gubitosi (aka Robert Blake)
Was a regular member of the Our Gang shorts when he was a kid as Mickey.
And was the lead in the series Baretta as an adult.
My all-purpose answer: Don Hastings, who played the Video Ranger on Captain Video at age 15, and is still acting today on As the World Turns. I don’t believe Hastings has gone more than six months without being on a TV series.
Shelly Fabares was on The Donna Reed Show at age 14 and a regular on Coach in her 40s.
While not quite a child, Genie Francis started on General Hospital as Laura when she was 14 and had that role for 26 years (1976-2002). Bonus trivia, she and Jonathan Frakes will have their 20th wedding anniversary in May.
Brian Austin Green was on 90210 as a child/young adult (starting at age 17 so just makes the cutoff) and was a regular on Ressurection Blvd and Freddie as an adult. He’s now a semi-regular on The Sarah Connor Chronicles.
Jenna von Oy in Blossom and The Parkers
Elinor Donahue was on “Father Knows Best” as a teenager and “The Andy Griffith Show” and “The Odd Couple” as an adult.
One that maybe just barely counts is Butch Patrick, who played Eddie on The Munsters and then starred in Lidsville. Although he was presumably playing younger, he was about 20 when the second series ended. Although he “retired” from acting shortly afterward, he still pops up in small parts and cameos from time to time.
Susan Dey was a teenager when The Partridge Family first went on the air. She was later a regular on L.A. Law.