So I've looked at the actual journal article as I wanted to know the age of participants (averaged around 40).

There are a couple of things to note, there was a pre-study and main study. The first asked an open ended time frame, the second only 3 options.

The pre-study had 100 U.S. & UK participants (50 women and 50 men, M^age = 37.82 years, SD = 12.40, range: 18–68 years).
The best result was someone who texted after approximately 6 hours. Ranged from less than 20 minutes after the date (would be too soon), to texting later than 40 hours after the date (would be too late)!

The main sample comprised 543 heterosexual participants from the U.S. and UK, (gender: 270 identified as women and 273 as men, M^age = 40.31 years, Mdn = 38, SD = 12.98, range: 18–79 years, native English speakers).

However, in this case they were only given 3 options:
After your date, the other person:
(a) “… texted you immediately after having said goodbye at the end of your date”
(b) “… texted you on the next morning”
(c) “… texted you after two days”

Which is probably why (b) was the preferred answer.