think even human have problem with age. 25 year old with home and kids not more 25 than one still in school. not like these number mean anything more than years in world.
with faery possible more complicate. have even less pressure, less things give goal and meaning, and also different life span.

we can act ways human think child, but what really make child? wonder? joy? play, dance, sing? or maybe feelings, how can not control same way?
those not child to us. those natural, for all age. even adult who see whole world still should have wonder in heart, laugh and dance with wind, jump in shallow water…

some faery child forever. some never child. some grow in way humans understand. some seem change with day.
age not so easy. some not remember or care. some do. but many not look how expect with this in mind.

90 summer old who hardly look over 18. 5 summer old who stay this way for 30 summers. 300 summer old who still proudly "think like child", but not one. 2 summer old look and think like wise old man.
same age for two different faery can look totally different - numbers not really help much. unlike humans, no standard for how "should" age.

adult, child, infant... those words do better explain age than numbers. even if someone maybe develop more or less than "should be" , that not make wrong how feel. 
no one say am less adult just because need help in ways some children might. and no one say children any less child just because they can recite shakespeare from heart.

our bodies reflect age that feel in moment. for some this means change each day. for some, never do. for some, change in straight line like humans expect. and for some, flip wildly.
still have things that only for adults, of course - rituals, places, conversations. but one can be around for 500 summers and still not adult. number not good measure for that. body, mind, and actual time just not always match.