Can't remember when my interest started coz it has always been there. I remember being as young as 4 or 5 and gazing at the night sky regularly when I was allowed to stay up late (late being about 10pm
)
I didn't know the names of things but I did know the patterns in the sky ( I didn't know they were constellations), and watched partial Lunar eclipses and meteor showers as a tiny kid, the obligatory Moon Landing on TV at school etc etc. The fascination has never left me, the only difference is that now I use a telescope rather than just my eyes alone.
So I suppose my interest came from just seeing the sky from a very young age and being fascinated by it.
P.S. I am about to turn 50, I have been into Astronomy my whole life, and I STILL don't know the constellations!!!
Sad but true.