There's only one 'first' first solo - in my case JP3a XM412 at Linton's RLG of Church Fenton in 1977 (immediately before the aircraft disappeared into ASF for a major service. Went in a month later to have a look, saw how puny the main spar and tailplane components actually were and resolved to try a bit harder not to be so ham-fisted with my aeros. Or landings. Or anything). On reading my 5000 series many years later it transpired that my instructors had noted my underconfidence and used a procedure they referred to as 'covert review' to justify the flex that I clearly needed. Eternally grateful to them; never heard of that system of review when I later became an instructor.
First time I was let loose in a helicopter was later that same year - Whirlwind HAR10 XR453. Still grateful to Oldbeefer - it was the start of 42 mostly-very-happy years in the best job anywhere in the world.
If talking about anything aviation-related, then the first solo experience would have to be the sunset static-line drop under a double-L canopy at Headcorn a few years earlier.