How old were you when audio gear first caught your interest?

I was 8 or 9. I kinda inherited a mismatched pair of Singer(?) tube integrated mono amps my uncle had built from kits.  I built my first set of speakers shortly there after.  My mom had a turntable and a ton or records, so as long as it was symphony or show tunes I had plenty to listen to.

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@elliottbnewcombjr - Memories of the wild-side they left out of American Graffiti.

I was given a electronics kit, around age 8. I made a receiver and then also a transmitter.  It was all clip together, no soldering needed. Next, I bought small portable radio / cassette player, Sanyo maybe?  I hooked the headphone jack up to my Dad's Traynor speaker cabinet, and it worked! 2 12" woofers, bass ported.  No one thought it would work, but, surprise.  I had to make the headphone jack to 1/4" jack adapter.  HOOKED.. after that, I did a paper route and then saved up my money to buy the Telefunken M242 Reel to Reel, V250 integrated amp and T250 Tuner.  I still have the amp and tuner. I was 15 years old, with 1 of 250 special order units, likely the best Telefunken ever made. 

I grew up in a household with no music until was about 16, then my dad bought a console "stereo" with  a disc player - on which he listened to show tunes.  I am not sure where my mother developed her distaste for music, her father had been a violin teacher, among other things, but he was killed in WW I when my mother was only 4.  I can imagine that listening to beginning violin students would be detrimental

GB Shaw famously said of a violin performance "difficult, I wish it were impossible."

Fortunately for my sanity the local youth club to which I belonged had fairly frequent trips to London for weekend afternoon concerts at he Royal Albert Hall, the Royal Festival Hall - or sometime to recital halls, so I was exposed to great music, live, hearing the great performers of the late '50s and early '60s.

In my teens I became a "Short Wave Listener" - I listened to the ham-radio bands on an R1155 receiver.  That was the receiver installed on WW II Bomber Command aircraft.  I could also listen to music, on headphones, through this monster device.

That was a long winded way to say that by the time I was 18 I was totally hooked on classical music.

Financial constraints limited my audio gear to a small mono radio until perhaps 1968 when I was 24.  After listening to various systems I purchased a Quad 33, Quad 303, a pair of Quad ESLs, fed from the (inevitable) Garrard 301 + SME 309, with a Shure V15 Type II.

That system was lost when I moved to the US in 1972 and years of stereo receiver + decent but not high end speakers persisted until the High End journey started again in about 1995 with a system based on a C-J MV 52 (which is still in my basement) and a used pair of Quad ESL-63s (which blew up long ago).