Under 22 years old comes to mind for some reason. Not sure why or what makes me draw the line there. However, I was suprised when I was in hospital recently in pre-term labour and I overheard one of the nurses referring to me as the "young Mum" in Room 12. I'm 29 but it felt nice to be referred to as a "Young Mum". So, obviously everyone has a different view on this.