What you'll find is that it varies dramatically by the child and their sleep habits. Some children are sound sleepers who rarely move; some are rollers and will go all over, making adult-height beds challenging due to them falling off easily.
Both of my children were in "regular" beds by 3 or so. However, one of them is no longer - he now mostly sleeps on the floor, but that's by choice - he has a bed, he just likes the floor. My oldest is mostly a sound sleeper who stays in roughly one place; he rarely is out of his bed. My youngest, though (at 7) is a roller. He will go from one end of the room to the other, and if he does start on a bed odds are he's stretched out horizontally on it, and will slowly slide down to the floor then start moving on that. For him, we needed to make accommodations; we let him use the trundle bed, which makes rolling off much less of a concern, especially at 3 or 4. (He still uses it, mostly, though sometimes on an adult bed now.)
My suggestion is that if she's fine sleeping on your bed, she may be able to sleep on an adult twin bed; but make sure it's in one corner of the room, with two sides on the wall, and arrange things so she tends to be on that side of it. If you find her falling off, then you may need rails, but some kids are fine without - it just depends.
To test it, you could try using the crib mattress on the floor for a few days, and see if she's able to stay on it during the night. If she mostly is, then she might be a good candidate. You can also use the crib mattress below the bed when you do try that - a soft landing space if she falls off, or just wants something closer to the ground. Kids are soft and flexible and generally okay sleeping on the floor, after all.
As far as "the talk", the biggest challenge for us was convincing them to stay in bed, and understanding that we have to be tolerant of them walking about some - they're no longer confined to a small space. Having to learn to accept that and not jumping on them every time they're out of bed was hard, but once we learned to let them settle down by having that freedom, it was fine.