You can refer to actual photos of boys and girls in that age range and draw from them, observe and draw your own conclusion. Normally the younger the person is, the eyes and upper face are relatively larger. The more apart the eyes are indicate the age too. But if you're stylizing it, like all your characters' eyes are large, then I dont' have a simple way of explaining it. So my best idea is to look at actual references, and also look at your influences to see how they depict these age groups.

Hmm…certain facial features help highlight the age, the expression on the face too. A young boy aged 10-15 may have a michevious or young refreshing look to him, or an apathetic glare common with teenagers. A lady in her early 20s may have some makeup on or hair style popular among the age group. Maybe facial features of maturity and a strong sense of self. I’m no professional, just my objective opinion on this. Hope this helps🤲