Um, I would say after college, but honestly, I am 30 and I just recently started liking stuff in the women's section. I think I do tend to look young, though, so up until now, only juniors stuff looked normal on me. I mean when it comes to jeans and a t shirt or a simple dress, there's not a big difference.
I think the sections don't really matter. It's all about what looks good on you, is appropriate for you life, and you feel comfortable in.
I'd say after you're sixteen it's acceptable but it's up to you, your own pace is what suits you best so there's no pressure. However in my opinion, at 23 you should be wearing clothes for your age, great that you can still fit in the teen wear but for your own sake I'd start shopping there next time you need something. Lots more choice too!
Dont shop in the women's section until you're like 35. At 23 you can wear some juniors clothes but you can get into the misses section. Misses is perfect for mid 20s early 30s, a lot of women's clothing is too matronly