Great topic, Shaj! I almost can't answer it, as I never really sarted collecting as an adult, I just continued my teenage collecting. I've been working pretty much full time since I was 16 years old (back in '96) and as such, I've had a decent amount of disposable income for a very long time.
Anyway, my collecting as and "adult" started not with Transformers, but with Star Wars. The old Micro Masters Action Fleet line to be exact. Also got pretty heavy into the Power of the Force line. I, too, remember getting those Han and Luke stormtrooper figures, thought I don't remember exactly how, aside from some sort of mail in. I don't have my Action Fleet stuff anymore, I lost it all in one of my moves back in the day, but I do still have all of my PotF figures in deep storage. Ah, the memories.
I dabled a bit in Beast Wars at this time, but kinda ignored the line as I took the "truck, not monkey" line of thought at the time, and never gave it a chance. Honestly, I only bought Transmetals Rhinox and Waspinator at the time because I liked the vehicle alt modes. Man, I'd like to go back twenty damn years and kick some since into teenage me. So Transformers wasn't on my radar at all.
Then one day, while picking up my weekly pull list at my comic shop, the cover of a Toyfare issue caught my eye (for another toy, I forget what), and I picked it up. Inside was an article on the upcoming RiD cartoon and toyline. Goddamn, but I was sold in a heartbeat. I asked Casey (dude who owned the comic shop) if he was going to get these toys, and he said yeah, so I had him hold the figures from his first shipment for me.
What were those figures? RiD (2001 for the kids in the room) Leader class Optimus PRime and Ultra Magnus. Man, let me tell you just how freaking awesome these figures were to a young nerd who had virtually ignored Transformers for the last decade or so. I made it a point to record the cartoon, got all the figures, discovered Beast Wars on Foxkids reruns and I've been pretty much all in ever since. Granted, after revenge of the Fallen came out, I reduced my collecting to mainly Masterpiece figures, but I really haven't stopped collecting Transformers since 2001.
One day I may grow up, but don't hold your breath.