Exactly the same for me.
Started collecting pretty stones from the garden, walks, and vacations from age 5.
This later as a teenager became a mineral, gemstones & fossil collection - which I still have today, although don’t really collect anymore. Half of this collection is in a refurbished metal cabinet with drawers, and the other half spread across my living room - most of which in a glass cabinet.
I’ve also collected post stamps from age 6. I took over my mom’s, uncle’s and grandma’s collections when I was 14 or so. Stopped that collection a few years later, although I still tried to get all (Dutch) Christmas stamps of every year. I think I have all Christmas stamp sets from 1989 to at least 2018/2019 or so. After that I still ripped the post stamps of letters we received, but no idea if I have all of them for all the years after that.
That collection is mostly collecting dust in my attic now. But it’s still cool to have a collection spanning three generations. 
Me and my little brother (with help from my parents of course, or actually, mostly just my mom) also collected loads of Dutch supermarket collectibles: Flippo’s (aka pogs); Happy Tails (aka a brand of stuffed animals), including their Christmas finger puppet versions; FoxKids/TV cards; Pokémon pogs; Yu-Gi-Oh pogs; 3D animal cards and 3D-glasses; etc. etc.
Apart from the regular Flippo’s, which I threw away during a cleaning once (kinda regret it), I still have all of these little collectables (somewhere).
As for TCGs, I started collecting Pokémon TCG cards at age 7, until roughly age 15 or so. Yu-Gi-Oh from age 11 until maybe 16, and then again from age 21, up until I started collecting Pokémon from age 24. Now, turning 34 two weeks from now, I’ve been collecting Pokémon TCG cards for roughly ten years; as well as non-TCG Seviper stuff for roughly 4-5 years; and one of each Yu-Gi-Oh rarity for about 4-5 years as well.
Apart from Pokémon, Yu-Gi-Oh, and those earlier mentioned supermarket cards, I’ve never collected any other card games.
One other very big collection of mine was twisty puzzles. Started that collection near the end of 2012 (at age 22, let’s stay consistent in mentioning those
) after asking for some twisty puzzles for Sint Nicolas (Dutch holiday similar to Christmas), since I didn’t knew anything else to ask. After enjoying those, I started buying A LOT myself, and the collection rapidly grew. Collected those until 2019 (age 29) or so, when I was focusing more and more on Pokémon. I have sold some of my puzzles since then, mostly the ones I didn’t like very much, but I still have over 400 twisty puzzles left - some of which I’d still tried to sell (all those puzzles are still in their moving boxes since I started living on my own though - and the book cabinet they used to be in now holds my Seviper non-TCG collection
).
I’ve also had some digital collections within games, like trying to get all weapons in TF2 (except for the Golden Frying Pan / Wrench for obvious reasons); or the best level 19 twink gear for WoW or level 15 twink gear for FlyFF. 
As to answer your questions.
- “Why do you think that is?” No idea why tbh. I always liked it, from a very young age, and I’ll likely always will.
Maybe it’s also in my genes, since my grandma on my mother’s side also collected a lot. Like the post stamps she gave me, but she still collected Swarovski crystal figures, and mouse figures (my grandparent’s last name roughly translated to Mouse Mountain, so they decided to collect those one day) in her old age.
- “What is your next collecting journey outside of Pokémon?” Currently only one of each Yu-Gi-Oh rarity, including misprints. But apart from that, I’m much more likely to start another sub-collection within the Pokémon TCG (or continue Pokémon TCG sub-collections I’ve stopped over time), then going to a new collecting community entirely. If it weren’t for financial/time restrictions, I would have already started 10+ other sub-collections within the Pokémon TCG, haha.

- “Are there collectible circles that you would never consider joining?” Metazoo and Lorcana. First one, because I feel like it’s mostly been introduced as a money grab, and unlike Pokémon/Yu-Gi-Oh, I don’t have any nostalgic feelings to it. And the second one, because it’s just not for me. I do enjoy some Disney movies, but collecting cards of it, nty. Especially not for similar prices as Pokémon/Yu-Gi-Oh cards.
And two answer two other questions of my own:
- What other collectible circles have you debated about joining? Both the Harry Potter TCG and MTG have some pretty cool cards. I have kept cards from both those TCGs when I randomly received them as gifts with Pokémon orders. But although I’ve considered collecting both more than once, I doubt I’d ever collect either at this point tbh. As I mentioned before, too many collection goals within the Pokémon TCG to consider these.
Having said that, I have bought some of the missing holo FoxKids/TV cards a few years back on impulse, and made a list of the missing holo/WINNER cards to fully complete it, but I’ve stopped right after that first order arrived again. Unlike the Pokémon TCG, even though I never intend to sell my collections, it still felt like a total waste of money to spend 10-25 euros on random FoxKids/TV lots/collections, just to get a few new holos. I likely had to spend close to 300-400 euros to fully complete it, which is just not worth it tbh, even though they did give me nostalgia, and it would have been fun to own all cards. (Although I wasn’t even able to find a full list of all WINNER-stamped cards available, nor the code variants some cards had, so I’m not even sure which cards there are for these in order to own all of them to begin with.
)
- How does collecting Pokémon compare to other collecting journeys you’ve had in your life? When I started collecting twisty puzzles, I really liked how besides just collecting, you could also solve them. That’s why I enjoyed collecting those for quite a few years. When I switched to Pokémon however, one big advantage, was that you had goals to work to, instead of just buying new collectibles (twisty puzzles in my case) endlessly. Choosing Pikachu as a collection goal might have been a bit of a counterpoint in that regard, ngl
, but I still had a goal to work to and try to complete. Likewise with all my other Pokémon sub-collections (except maybe Seviper non-TCG stuff). Every one of them has an end goal to complete it.
And as mentioned before, this amazing community is also a big reason why I still enjoy collecting Pokémon stuff for such a long period of time.
Greetz,
Quuador