Why not several different actors?
That'd be confusing, if they intend to have a traditional biopic. Depending on how he's covered, the usual course of action would be one child actor, one teenage actor, and one adult (30-ish) who can play Roddenberry from about 20 to his death (at age 70).
But, to be honest, I don't think there's any pressing need to cover him before the 1960s. I mean, yeah, there's interesting WWII and pilot stuff, but the meat and potatoes is his television career, from his quick rise to fame in the '60s, to his dip in the '70s and his rise again in the '80s. A pretty good three-act structure right there. Family life and earlier careers can be mentioned.
But, you know, the big worry is that this is a noted philanderer who explicitly used his power to pick up new girlfriends who worked for him. The movie is going to treat him as a hero, even if they don't whitewash his sins, and there's going to be alot of media pushback against that.
Maybe a good director can work something out, but I don't see how such a character can be survive in this day and age.