I don't know about "boundaries" as such, but I was very strong willed from a very young age.
Even virtually as a toddler I'd not be pushed around, knew my own mind, etc. I remember a couple of instances before I'd even started school (or maybe it's false memory being reminded of them by parents??). Once I was at a relative's house and they asked me what drink I wanted - I told them, but they gave me a different drink, so I just tipped it out over their carpet! Another time, we were having a family walk and my parents asked me to carry one of their coats (I think they were carrying something else), but I didn't want to, so they insisted and I grudgingly took it, then promptly hung it on a tree branch and left it behind and it took them ages to realise and one of them had to run back to fetch it!
I think that set the scene for the rest of my life really. I've never done what someone else wanted me to do if I didn't want to do it. I've never been "pressurised" to do anything.
I've walked out of a few jobs without giving notice etc when I've felt unfairly criticised, or bullied etc. Literally just picked up my stuff and left. Life's too short to accept being badly treated.
I've only had two serious boyfriends, both came along at the same time, so it was basically one after the other then back to the first! The one I "preferred" at the time, was constantly nagging me to do more "sexually" but I wasn't ready. We were going through the different stages as time passed, but he wanted to move through faster than me. I'd have got there in the end, but he was just impatient and he became a right nuisance with his constant pestering and taking liberties. He was never "forceful" nor abusive, always respected my pushing him away or stopping him doing something, but it just really irritated me the way he was always pushing my boundaries. I ended up reluctantly having to ditch him. I went back to the other who was the opposite - but I was so much happier knowing he wasn't going to try it on constantly - in the end, I had to be a lot more assertive to make things happen, but at least it was to my timetable.
Same has happened throughout my adult life really. Whether friends, family, neighbours, work colleagues, even clients/customers - I won't be bullied, won't be pushed around and will ALWAYS do what I want to do, when I want to do it. Like I say, started from a very young age so never really "learned" how to do it, more it was just nature/nurture to be my own person and no a people pleaser!