Marty aged 13 and 3/4
Self-portrait, February 1978.
OK, more like 13 and 2/3, but it fits in better with The Secret Diary of Adrian Mole Aged 13 3/4 if I say 3/4.
I didn't have a tripod at this point, hence the tilt of the world.
Details: Kodak Retina 1B (type 019), Kodachrome.