When did you move your child from babywear and onsies to big clothes? For us it was when he started toddle-running and the babywear couldn't take an active child. One too many ripped knees and not quite enough protection from grazes. It wasn't as easy to handle toilet accidents, but it was much easier to wash clothes than sew them up!
I started when my daughter turned ten months old as she started to be more active and wanting to walk. She would crawl and try to stand up and the onesies were no help for her so I opted to get her pants instead, thicker ones to protect her knees. I thought that pants were easier to change her as well since I didn't have to completely remove them unlike the onesies I had to remove the entire bottom part.
I don’t remember the transition from baby clothes to children’s clothes. From my recollection, it is our daughter who dictated the shift. When we are in the store, my daughter would be pointing to big dresses so that may have been the signal to shift from baby clothes to children’s wear. But I think in general, the parents decide on the kind of clothes for their children.
My son started wearing big clothes when he started walking. You are right @Taliska once they start walking or toddling around onesies definitely aren't durable enough. I think it's because of the thin material. Blue jeans or the little khaki dress pants is what he wore the most of and they seemed to last.
I think around 9 months or so is when the child may be safely shifted from onesies to more fuller clothes. Every child differs in learning standing and walking. It may be common sense application depending on the needs of your child. It is true that knee injuries are best avoided by tougher materials like jeans.
My son stopped wearing baby clothes when he turned 6 months old. By that time, he has practically outgrown them and I didn't want to buy more at that point. As for onesies, I was only able to make him wear them up to the end of his second month. He was too active to be confined in onesies. At home, he felt more comfortable with just baby tops,diaper, and pajamas.
I think it depends on several factors really like parents wallet, hand me downs from an older sibling, or even the child's weight. Some babies grow very quickly and cant fit into the tiny clothes by age 9 months. Some babies can still fit past the twelve months age. So I would say there is no blanket age month to make this switch. If the child is at home mostly (not attending a baby care center) this can be better managed. but if the child has to leave the home each morning, there is that pressure to buy more clothes.