Bobby Porter's Birthdate Correct on IMDb?

I'm a huge fan of the film, "Day Of The Animals" (1977), and was just looking at the cast info on IMDb. I noticed that Andrew Stevens was 22 years old at the time of this movie being released in 1977, and that the character of Johnny Goodwyn (who plays a VERY young looking 13 year old kid?), is listed as having a birthday making him 24-25 years old at the time of the movie's release!? How is this possible?