My oldest son is almost five and I admit he's not the most talented painter in his class. But no kid in his class could make something like that. But even older children in his school don't draw like this until they're about eight or nine. If Julian was/is a very talented young man, let's say he could have drawn this at the age of six or seven imo. Lucy In The Sky With Diamonds was recorded on March 1st and 2nd, 1967. John had the song finished by then, that's obvious. Let's say he wrote it somewhere in January or February 1967. Julian became four years old in April of that year. So he wasn't even four years old when he should have drawn that. No way.
