here where I am (NY the one in the USA as apposed to the New York in other countries) the inner child doesnt have an age. here the inner child concept means a person feels like a child. example someone getting married can feel like they did when they were 10 on their birthday or 5 when they went to kindergarten or 18 when they graduated high school. its all relative to a persons own life experiences and emotions they have felt through out their own childhood...

which means only you can say how old you were when you felt the same emotions you are feeling as an adult.

now if you are talking instead about alternate personalities ages. well that alternate personality will either tell you or they wont, they will either sign their name and age on things they own/draw what ever or they wont. theres no real way to determine an alternate personalities age unless they tell you, some people can match the alternate personalities memories to events in their life chronologically and sometimes that will reveal an alters age but doesnt always because alters can be anything any age in which was needed in order to survive the trauma they went through that caused the person to have the alternate personality.