Hard to say- I didn't know what a "furry" was until about middle school, which judging by these ages is pretty average, so about 12 or 13. It wasn't until I joined another forum for fantasy art fans that I really started calling myself a furry, after being surrounded by furry artists and fans.
I have always been very much into antro, but I didn't know what a furry was before i was 13. However, every time I was asked if I was a furry I denied it since I was already an emo kid with very few real friends so didn't want to make things worse for myself. I got more into it when i was 19 and went full furry 1 and almost 2 years ago. I don't ever regret going full furry, since I and those around me are older now they are much less judgemental and are often very curious about my interest. I've also met great people within the local and national community, some whom I've met in real life.
Um... 50. LOL
However, I've been a costumer for about 16 years, and although I've wanted to get into the furry Fandom for a few years, I finally commissioned a fullsuit ferret recently (Snickers) and have a partial Tabby cat (Pogo) that I'm waiting on his jacket to arrive.
11 was when it all started I'm pretty sure, at least that's when I made my first animal character that I saw as me. I've loved animal characters and anthros all my life though. Didn't get into the fandom as a poster until the age of 22, though. Long time in the making!
I've always been into anthro animals since I was a kid. I only started to know about the existence of furries around 2015 when Five Nights exploded, and many people were making fun of it by calling it furry. At first I only watched FNAF fan art on Deviantart back in 2015 and I would cringe at every anthro art on DA that wasn't freddy and his partners, I started to watch FNAF porn on r34 pahael. That was when I was accidentally by the luck of the universe introduced to MLP gen 4 Clop, I got used to it more than furry content because I was already familiar with MLP as my sister had an older gen MLP pony movie that I used to watch with her when we were younger. I would never become a brony or contributed to their fandom though and I would just lurk and fap to whenever an r34 artist had made clop. I would also masturbate to Spyro pronz which is the only scaly yiff I have masterbauted to. I then had a phase when I started masterbaiting to real horse/beastiality, I would watch some real ones and then I moved on to those 3D animated horse pron which seemed much more appealing. Then after a while and sometime in 2017 I stumbled upon a FNAF video which had the word yiff in it as a joke, and I didn't know what it meant so I searched it up. I discovered the yiff sub on reddit, and I became addicted to yiff like meth for a while. Why I didn't get interested in furry porn in my days of fucking fapping to fnaf porn on r34 back in 2015 I will never know.
Anyways I still don't really identify as furry or want to participate in the furry fandom either. I just prefer to be a lurker and observe the furry fandom so I guess that makes me a type of "ghost observer" member of furry. Besides, I just use furry sites for the porn.
I've been thinking about this further, and even though I became an official furry on 3/25/17, the seeds were planted for me being a furry during my adulthood when I first played Skyrim on 12/25/11, all the way back during my sophomore year of high school.
Me thinks 4th or 5th grade, some time around 2007. I didn't know what a furry was and assumed I was a freak and something was wrong with me until early 2014, when I discovered E621 [I can't say it was an accident, but some girl from the UK linked me to it], and then shortly afterwards I discovered FA and found I wasn't alone.