Our cot-bed instructions actually said to change to the BB at 18 months or if LO climbed out. Whilst many keep LO in a cot as long as possible I don't think it is at all unusual for them to move younger than this either.
My tip - a BGB safari. Which is lots of play on and around the BGB during A time. crawl around the perimeter on the floor, then climb up onto the bed, and off again, crawl around the perimeter on the mattress, roll back and forth etc etc...the idea is that the more they physically explore the parameters of the bed the better the mental map is, and it's the mental map which we all use to keep us in our beds at night when we sleep.
I put a few cushions and a folded duvet on the floor either side of DS's bed, he did fall out a few times but not as much as I thought and never seriously hurt.
I had a gate on his door and made sure his room was safe.
Good luck!