(Also agree with recode.)
A script for this might look something like this
dt = Open( "$sample_data/big class.jmp" );
dt << New Column( "group", character );
label = {"baby", "toddler", "toddler", "pre-k", "k", "1st", "2nd", "3rd", "4th", "5th", "6th", "middle", "middle", "middle", "high", "high", "high"};
For Each Row( group = label[age] );
Grouped ages
The if-then-else approach might be better if your data isn't contiguous, starting at one, or if there are only a few groups.
Craige