Seems to be working fine for me (using JMP16.2 Pro on Windows 10).
dt = Current Data Table();
dt << Select Where(:age == 12);
d3 = dt << Subset(Output table("a12", Selected Rows(1), Selected Columns(0)));
You seem to be missing : from the column name, maybe Toolbar is more strict on the syntax? At least if I remove that from the toolbar script I will get this error:
but it will work fine from script editor (in my opinion it shouldn't)
-Jarmo