[R] he^lp on "stack" function

[combe@b3e.jussieu.fr]

Excuse me.
I don't understant how to use the function "stack".
I'm not sure to know the good syntax.

I've got a data witch column names are the time when the mesures was done and
an
other who indicates the age of the subjetcs.
I would like to do a new matrix with patients older than a value and the new
matrix will contain in the first column the time of the mesures and in the
second th evalue of the mesure.

Could you help me please ?