If you lightly brush a solution of either black tea or coffee onto some good quality thick cream coloured writing paper, let it dry and then place between two heavy books (to smooth out the wrinkles), you will find that it *ages* the paper... Don't put milk in the tea/coffee and just use the minimum amount of liquid...
To get that real *aged* effect, use a proper fountain pen and make sure you use your bestest most gorgeous calligraphic lettering. A note using a biro and modern lettering will give the game away.