When I first got into metal depends on your relative definition of "into metal."
In 1996 when I was 12, I spent my allowance on Metallica - Load which came out that year, because I saw the Until It Sleeps video on TV and thought it was cool (yeah yeah, I know, I didn't know any better). I was later hanging out with a friend who was I think 15 at the time (knew him forever, our moms are best friends) and I told him about the album. He said "that's shitty new Metallica, you need to hear some of their old stuff!" He then played a few songs from the Black Album and Master of Puppets and I was hooked. Whenever I had the allowance and opportunity to go to the cd store I bought a new Metallica album, until I had all of them. I then checked out a few metal more bands like Megadeth, Pantera, Sabbath/Ozzy, and Tool. Throughout high school, I didn't add that many more metal bands to my collection, as I was listening to pretty much all forms of rock (including Nu Metal - I figured some of you would appreciate that distinction). I did consider Metallica my favorite band though.
In college, I became a huge jazz fan. I got really into playing music (alto and tenor sax) in both my school's band and some gigs at restaurants, took music theory classes, and attended jazz jam sessions when I had time. Jazz became my main indulgence (especially modal jazz, hard bop, and fusion). I then got into some progressive rock (Pink Floyd, Phish, Jethro Tull, etc.), which I was drawn to by the elements of jazz present in it. Subsequently, I checked out some progressive metal (Opeth, Mastodon, Dream Theater, etc.).
Throughout the rest of my 20s and early 30s (I'm 34 now), I was very eclectic, listening to many genres including metal, jazz, blues, progressive rock, classic rock, hard rock, alternative, hip hop, reggae, and so on, with jazz still being at the top. My metal listening mainly stuck to a dozen or so thrash, old school metal, and prog metal bands I was familiar with, except for a few times when I went on a metal kick and skimmed a bunch of bands over a couple weeks but forgot most of them and walked away with one or two that I stayed into (Amon Amarth, Kamelot, and Ghost came out these "metal kicks"). I would then get distracted before really gaining the metal familiarity/understanding that most metalheads I know have.
For the past month of so I have been on a huge metal kick, one much more intense than the previous ones. I think what triggered it was a chat about metal with a friend (who's a metalhead) over a few beers, when I realized that I enjoy metal too much to not be into that many metal bands and to not have really explored many of the different metal genres. I've been diving into a lot of metal genres, most notably melodeath, melodic black metal, and power metal, while dabbling in a few others. I feel like this metal kick is not going to be short and fleeting like the others as a lot of the new music I've been listening to has been REALLY clicking with me.
Back to the question of when I first got into metal, it depends if being a casual fan as I have been qualifies as being "into metal." If so, the answer is 1996 when I was 12 with Metallica. However, if "into metal" means being a full-blown metalhead, and if I continue my current metal indulgence on a permanent basis, years from now my best answer may be March/April 2018.