Dexter's Laboratory - Season 1 - Eps 9: Old Man Dexter
Banned from watching a late-night movie due to his young age, Dexter uses his technology to artificially grow into adulthood, but Dee Dee tampers with his machine, causing Dexter to turn into an old man.
About Dexter's Laboratory

Title: Dexter's Laboratory
First Air Date: 1996-04-28
Last Air Date: 2003-11-20
Status: Ended
Rating: 7.7/10 (from 813 votes)
Language: EN
Seasons: 5
Total Episodes: 220
Network: Cartoon Network
Genres: Animation, Comedy, Sci-Fi & Fantasy
Production Companies: Hanna-Barbera Productions, Cartoon Network Studios
Synopsis
Dexter, a boy-genius with a secret laboratory, constantly battles his sister Dee Dee, who always gains access despite his best efforts to keep her out, as well as his arch-rival and neighbor, Mandark.
Cast

Candi Milo
Dexter (voice)

Kat Cressida
Dee Dee (voice)

Jeff Bennett
Dad (voice)

Kath Soucie
Mom (voice)

Eddie Deezen
Mandark (voice)