I have never been hired as a professional personal assistant, mostly because I have never applied for the position, and I have only just started working full-time a year ago.

I have learned and gained experience as an assistant by working under my parent's family businesses: general trading and a small-scale restaurant.
When my parents were still in trade, they would be so busy that coming home would only mean them going straight to bed, which I then shared with them. I was very young (around 7), and I was taught to use MS Word and Excell to digitize their spreadsheets. The mails would usually be the same, so learning to copy-paste at that age was very helpful. Otherwise, I would only type out what they have handwritten.

They eventually let go of the trading business in the midst of running the restaurant. I helped out during my free times and vacation. From when I was 8, I would either be behind the cash register, plating different kinds of homestyle food, or bussing tables. It was fun. I sometimes helped my mother with her accounting. When I turned 16, I was more involved in the business. I also did stock takes, payroll, and some general shopping for both the house and the store.

I became proficient in MS Word and PowerPoint in high school, but I already knew how to work around it before we were even taught in school how to open the software because back then, the Internet wasn't exactly fast in our area so it's either my brother and I would play Space Pinball or The Sims on PC, or we would make weird ppt presentations where we would place a hidden clip art on the slide with a solid background, and it's either you get it right and be rewarded, or click on the wrong spots and get scared the heck out of the armchair.