Hmmm... that's a harder question than you'd think. The best time to start Heinlein is when you're a 12 year old boy. Or a reasonable approximation thereof. 12 year old girls are ok too. It works best when the world is shiny and new as your latest Scouting badge.
That said, his books are marvelous. My personal favorite is "The Rolling Stones". You might try one of his short story collections, probably "The Man Who Sold The Moon" or "The Green Hills of Earth". "The Moon is a Harsh Mistress" is a pretty libertarian piece of SF.... not written for kids... which you either love or hate. "The Door Into Summer" and "Double Star" are the other two novels that sort of sit on the cusp between his juvenile fiction and his weird stuff, IMO, and so make an easy read.
***not really seeing what the problem is here...***