Not sure just when, I knew fairly early about pets getting 'fixed' but wasn't sure what that actually meant... In Junior High I found a copy of Heinlein's "Farnham's Freehold" in the school library. Many characters in the book are 'tempered' and our main hero and his son get set up for 'tempering' early on but are spared (the son is done later in the book) EXACTLY what 'tempering' entailed wasn't quite spelled out, but it was pretty strongly implied by the descriptions... (BTW it was explicitly mentioned as being done at different ages, from infant to adulthood depending on their master's desires, and we met at least one character done at age 8...)
I won't go into the book further, beyond saying it is worth the read....
I actually found the book very 'disturbing' because I felt it wasn't 'normal' that I was excited by the concept, and actually got rid of my copy, it's the only Heinlein I don't own at this time... Later I encountered plenty of other literary descriptions, and exposure to animal castrations but was never enough of a history buff to realize how common it was historically beyond harem guards and opera singers....
It wasn't until a few years ago that I encountered a discussion of voluntary eunuchs online, and found it clicked enough for me to investigate further, which fairly rapidly got me to EAv2, and as they say "The rest is history"
WheelyFixed
Paraplegic - T-5, ASIA-B. 2010 Injury left non-functional & frustrated. 4/24/22, stop T. 5/4 start 3.75mg Lupron. 6/29 - T ~0. 7/7 - start E. 9/2 stop Lupron. 3/30/23 - GOT LETTERS! surgery (O&S) 9/28/23. Doing 0.75mg/day E patch as HRT