So I guess I collected the fully-plastic G3.5 seaponies (mer-ponies?) when I was about...well they came out in 2010 so I must've been 6.
While I always liked playing with the real and fake ponies I had, I never really had the urge to collect regular G3s at the time (oof) but after getting the underwater castle playset that came with a Pinkie Pie (probably for my birthday) I just really wanted to get the rest of the mer-ponies. A friend gave me a chewed-up seapony Cheerilee but it was better than the cardboard cutout of Cheerilee I was using, heh.
That Hanukkah my parents ended up getting me two other sets with them so I had Sweetie Belle, Star Song, Toola-Roola, a new Cheerilee, a Scootaloo, and another Pinkie. I was ecstatic! Played with them a lot. I was never able to get a hold of Rainbow Dash tho, it's the bane of my existence...
...anyway, during the super early years of FiM my little sister was given some ponies but she would maybe run around with it for a day, lose it, and then forget about it. We would willingly get pony merch and storybooks sometimes, but not ponies themselves, we just weren't all that interested, I guess we just liked the show and that's it, lol. But then around 2013, we both discovered these YouTube channels where people would film little series and stories with their ponies. MLPStopMotion was our favorite and the channel that made us want to collect ponies. We went to Toys R Us with my $40 dollar savings and bought some ponies (Twilight, Rarity, Pinkie Pie, Princess Celestia, probably more I don't remember). We also eventually found the ones my sister lost around the house (Mrs. Cake, Twirly Treats, Flitterheart, talking Princess Celestia, and a 12-pack of minis.) We played with them a lot. Been collecting G4 ever since, and I really want to start collecting the previous gens too!