Formula to find age groups

oops sorry try this array confirmed with control shift and enter

=SUM((B1:B50>=23)*(B1:B50<30)*1)/COUNT(B1:B50)%

assuming you have no zero values
count(b1:b50)could simply be replaced by the numer of rows or if you have zero values replace it with

=SUM((B1:B50>=23)*(B1:B50<30)*1)/COUNTIF(B1:B50,">0")%

still an array