The Chemotaxis Kit says to use 12-18 day old cultures. Does this mean I should sow the spores 12-18 days in advance, or does the 12-18 day age indicate the number of days after the gametophytes germinate?

The timing refers to the age of the cultures, day 0 being the day the spores are sown. Under the recommended conditions the spores should germinate beginning at day 3 and then gametophyte development will be quite rapid. At 28-30 C and under constant light the gametophytes will be sexually mature by day 12 and the chemotaxis can be done any time from then until about day 18. Be sure to note that when the cultures are removed from warm culture conditions to room temperature, the sperm will release from the antheridia even in the absence of liquid, so students should have all the preparations made prior to obtaining the cultures.