Sir Paul McCartney marked a poignant family milestone this week and chose to share a rare photo from his childhood with fans in honour of the occasion.MORE: John Lennon and Paul McCartney's sons look exactly like their famous fathersThe Beatles icon, 79, posted a black-and-white snapshot showing him as a little boy on the beach with his mother.WATCH: Paul McCartney serenaded by daughters during birthday celebrationLittle Paul can be seen sat on a blanket by his mum's feet wearing his swimming