In general: since I was 5 years old, and I haven’t stopped since (I’m 28 now). I started collecting peddles and post stamps at that age, and I’ve collected LOADS of things in the past (and still do), like: minerals, gemstones and fossils; post stamps; Pokémon TCG cards; Yu-Gi-Oh TCG cards; collectibles from local supermarket (like Fox Kids cards; different type of stuffed animals; flippos/pogs; marbles; etc. etc.); twisty puzzles; coins and euro-coins; and even virtual collections, like one of each TF2 weapon (except for the extremely rare ones like Golden Frying Pan); Smite voice packs and skins; Overwatch cosmetics; FlyFF most OP possible twink lv 15 set; World of Warcraft pets; etc.
As for Pokémon TCG cards in particular: I started collecting them at the age of 7. Would receive a pack every now and then for my birthday, Christmas / Sint Nicholas; good grades at school; or would buy them myself after I received money from my grandparents. I stopped around the age of 14 or so. I’m not entirely sure anymore what I primarily was collecting between age 14 and 18, but at around age 18/19 I started playing and collecting Yu-Gi-Oh pretty actively again, and bought loads of cards that I had decks of at online games like DuelingNetwork (even though I only ever played the game irl with my little brother…) I remember primarily buying from GametimeCC back then, until I discovered TrollAndToad, which had a much wider stock available. I noticed they were also selling Pokémon cards, so I bought the cheapest Venusaur, Hitmonlee, and Nidoking I could find once, which were the last three Pokémon missing from the first 151 Pokémon from my collection as a kid.
And then at the start of 2016 I placed all cheap Pikachu cards TrollAndToad had in stock in my cart, and that’s how my Pikachu collection started. 
I started looking which Pikachu cards existed, and also started buying foreign Pikachu cards. Then in the Summer of 2016 I came across justinator’s thread here on the Efour forum, who had loads of Pikachu cards in his collection and wanted list I had never even seen before. That’s when I joined the forum, and I was really inspired by Justin’s collection. Near the end of 2017 I surpassed him in terms of largest Pikachu TCG collection in the world, at which point I had around 700 unique Pikach cards. I now have over 1300 unique Pikachu cards in my collection, and also loads of other Pokémon sub-collections (with over 11,000 unique and 9,000+ duplicated Pokémon cards right now) - I no longer have the largest Pikachu TCG collection worldwide anymore though, since a fellow collector and online friend of mine from Belgium surpassed me least year, who has around 30-50 more than me.
Right now I primarily collect Yu-Gi-Oh rarities (TCG, OCG, Korean, and misprints); Pokémon Full Art Supporters in English and Japanese exclusive; all languages Seviper cards; and the WotC #21 Moltres promo artwork in all variations (including sealed and PSA/BGS graded). My Pikachu collection is on a slightly lower priority now, since in 2019 alone more than 400 new Pikachu cards were released, so I need to get my bank account back up a bit first… 
I also collect (also on low prio right now): the full National Index including all forms and Shiny Pokémon; twisty puzzles; and euro coins. Post stamps and minerals/fossils I don’t really collect anymore, although I still have those collections.
Greetz,
Quuador