Depends on what kind of cot u have and whether the height of the rail is high enough to prevent baby frm falling over. Generally parents wld be more concerned once baby starts to stand. Crawling shld still be ok. My 5 mo baby just started crawling but he's still safe in his cot and playpen. He's always rolling and moving ard in them but nvr went over the railing as it's much higher than his body when he's on all fours. My cot is an adjustable one tt can be lowered to become a toddler bed so I wld lower it once he starts to stand up.
3½ years.