There aren’t all that many 23-year-olds directing...

There aren’t all that many 23-year-olds directing full-length features--but Charlie Matthau claims it was his background as a studious film maker, not bloodlines to father Walter, that put him at the helm of “Doin’ Time on Planet Earth.” The $3-million pic is shooting for Cannon Films--for whom Matthau Sr. starred in “Pirates” and may do a “Born Yesterday” remake.

Matthau Jr. says the nepotism issue will be a “moot point” when “Doin’ Time” is released, tentatively next summer.

“I’ve been making Super-8 movies and working on my father’s films since I was a kid,” said Charlie, who made a student film, “I Was a Teenage Fundraiser,” at USC, which he claims got him the Cannon job.

This film is “about alienated teen-agers. I went to school with a lot of them and was one of them.” Adam West and Hugh O’Brian star, and Walter won’t even be cameoing. “Can’t afford him.”