Beetlejuice
Cartoon Series: Season 01; Episode #01: "Critter Sitters" (09-Sep-1989)
After starting to babysit, Beetlejuice likes the baby so much he says "it makes me feel like a kid" and he turns into a baby til near the end.
-- Tazz
Beetlejuice
Cartoon Series: Season 04; Episode #34: "Time Flies" (24-Oct-1991)
Beetlejuice and his "12-year-old" or young adult companion (age undeterminable from continuity or animation style) Lydia Deetz go to a time land and Beetlejuice turns into a baby and the girl becomes an old woman for a minute. First-season throwaway reference to being in seventh grade. Assuming time passes in the show as in real life, she would be in her mid-teens by the time of the series finale.
-- Tazz
Beetlejuice
Cartoon Series: Season 04; Episode #38: "Snugglejuice" (30-Oct-1991)
Beetlejuice is put on trial for a crime he didn't do he says "I'm as innocent as a new born baby" and he becomes a baby for a few seconds.
-- Tazz
Beetlejuice
Cartoon Series: Season 04; Episode #50: "Beetlegeezer" (15-Nov-1991)
Beetlejuice makes himself an old ghoul and hangs out with the girl's living grandmother. At the end him and the girl's father start crying and he says "You can call us cry babies" and they turn into babies.
-- Tazz
Beetlejuice
Movie (1988)
Ghostly couple are aged toward the end of the movie.
-- Jeffr_2bya